Monster Cookie Bars (Thick, Chewy and Ready in 45 min)

Everything you love about oatmeal cookies, peanut butter cookies, M&Ms and chocolate chip cookies - baked into one thick, chewy Monster Cookie Bar. One bowl. No mixer. No rolling. Just cut and serve.

Monster Cookie Bars are the thick, chewy, all-in-one dessert bar that combines everything you love about oatmeal cookies, peanut butter cookies, and chocolate chip cookies - all in a single pan. If you've ever wanted to skip the scooping and rolling but still land that bakery-style cookie flavor, this is your recipe. Packed with creamy peanut butter, old-fashioned oats, mini M&Ms, and semi-sweet chocolate chips, these bars come together in one bowl with just 20 minutes of prep.

Bonus: swap the M&M colors for any holiday and these monster cookie bars from scratch instantly festive. More on that later - so keep reading.

Monster Cookie Bars (No-Chill, Extra Thick & Chewy)

Soft and chewy monster cookie bars are loaded with peanut butter, oats, chocolate chips, and colorful mini M&Ms. Hey, they are just a little bit over-the-top ... well, maybe a lot, but in the best possible way.

These easy monster cookie bars are the easiest way to get everything you love about monster cookies - without rolling a single ball of dough. Whether you're baking for a crowd, a bake sale, or just need a no-fuss dessert on a busy weeknight, this is the recipe you'll reach for again and again. And if you're wondering how these compare to regular monster cookies - the bars are just as chewy and flavorful, but so much faster to make.

They are also irresistible just like Oreo Chocolate Chip Cookies and Strawberry Crumb Bars. Every bite is a perfect balance of sweet, salty, and soft crunch, with melty chocolate and bursts of candy-coated joy. Perfect cookie bar recipe!

Why you'll love monster cookie bars

• Time-saver compared to cookies: When you don't have time to roll and bake a few dozen cookies, just make a sheet of monster cookie bars instead. They are an easy go-to when on a time crunch!
• Flexible recipe in terms of ingredients: And you can use whatever you have in your pantry: dark chocolate chips, peanut butter chips, white chocolate chips, or even chopped Reese's. If you're a peanut butter and chocolate fan, my Fudgy Buckeye Brownies belong on your list too!

Monster Cookie Bars Ingredients

• Old-fashioned rolled oats: I find that old fashioned oats provide the ideal chewy texture and structure to hold all the mix-ins in the monster cookie bars (as opposed to quick oats). You can also use a mixture of old-fashioned oats and quick oats.
• Mini M&Ms and semi-sweet chocolate chips are great add-ins for the monster cookies. You can use any other coated candy or try something like chopped nuts, butterscotch chips or mini pretzels to switch the flavors up!
• Creamy peanut butter: For best results use regular creamy peanut butter. Peanut butter is also the star of my No-Bake Avalanche Cookies if you want a no-oven option.

How to Make Monster Cookie Bars

This is an overview with step-by-step photos. Full ingredients, measurements & instructions are in the recipe card below. 

Step 1. Mix in Wet Ingredients

Make sure your eggs, and peanut butter are at room temperature and your coconut oil is completely melted. This helps everything blend smoothly for an even texture.

In a large mixing bowl, add the brown sugar, white sugar, peanut butter, coconut oil, egg and yolk and vanilla extract. Whisk until well combined - about 30 seconds.

Step 2. Mix in Dry Ingredients and Make Cookie Dough

In a separate bowl, whisk together the dry ingredients: the flour, baking soda, oats and salt.

Add dry ingredients to the bowl with wet ingredients. Take care not to over mix at this stage. Over mixing can make monster cookie bars tough rather than soft and chewy.

Step 3. Add in the Mix-Ins

Tip in the M&Ms and chocolate chips and fold in gently. The dough will be thick.

Step 4. Bake and Cool

Spread the dough evenly into the prepared baking pan, pressing it down gently with the back of a spoon.

Bake monster cookie bars for 25 to 30 minutes. The edges should become golden and a toothpick inserted into the middle should come out with just a few crumbs.

Let the bars cool in the pan to avoid crumbling. They slice much more neatly once fully set.

Tips for Thick, Chewy Bars (Don't Skip These)

Use old-fashioned rolled oats, not quick oats.
Quick oats absorb moisture faster and turn soft and cakey during baking. Old-fashioned oats hold their structure, giving you that satisfying chew and slight bite in every bar. It's a small swap with a big textural difference.

Don't melt your butter - cream it.
Melted butter produces a flatter, denser bar. Softened butter creamed with sugar creates tiny air pockets that give the bars lift and a slightly crisp edge while keeping the center thick and fudgy.

Use conventional peanut butter, not natural.
Natural peanut butter separates and introduces excess oil into the batter, which throws off the fat-to-flour ratio and produces greasy, crumbly bars. Jif or Skippy-style peanut butter is stabilized and gives you consistent results every time.

Do not overbake.
Pull the bars when the center still looks slightly underdone - it will firm up as it cools in the pan. If you wait until the center looks fully set in the oven, you've already gone too far. Target 22-25 minutes at 350°F, and start checking at 20.

Let them cool completely before cutting.
We know. It's hard. But cutting warm bars compresses the layers and turns your clean squares into a crumbly mess. Give them a full 30-45 minutes at room temperature - or 20 minutes in the fridge if you're impatient.

Press extra M&Ms and chocolate chips on top before baking.
Reserve a small handful of mix-ins and press them into the surface of the batter just before it goes into the oven. This is purely visual - it gives the bars that bakery-style, loaded look in photos and on the table.

Pressing a few extra M&Ms and chocolate chips on top of the dough before baking makes the monster cookie bars look bakery-style and visually appealing.

If you want to, you can also scatter a few more M&Ms when the bars are hot from the oven to make these monsters even more irresistible!

Frequently Asked Questions

Can I use quick oats instead of old-fashioned oats? You can, but your bars will come out softer and more cakey rather than thick and chewy. Quick oats are more finely milled than old-fashioned rolled oats, which means they absorb moisture faster and break down more during baking - essentially disappearing into the batter instead of contributing texture. Old-fashioned rolled oats hold their shape and give the bars that signature chewy bite. If quick oats are all you have, reduce the baking time by 2-3 minutes and watch carefully, as they brown faster.

Can I use natural peanut butter? I don't recommend it. Natural peanut butter - the kind where the oil separates and you have to stir it - has a much higher oil content and lacks the stabilizers found in conventional peanut butter like Jif or Skippy. When you bake with it, that excess oil throws off the fat ratio in the batter, resulting in greasy, crumbly bars that don't hold together cleanly when cut. Conventional creamy peanut butter gives you consistent texture, better structure, and that rich, familiar peanut butter flavor in every bite. Save the natural peanut butter for toast.

Why are my bars cakey instead of chewy? There are three likely culprits. First, overbaking - cakey bars are almost always the result of leaving them in the oven too long. Pull them when the center still looks slightly underdone; it firms up as it cools. Second, quick oats instead of old-fashioned oats - as noted above, quick oats absorb more moisture and produce a softer, cakier texture. Third, too much flour - if you scooped your flour directly from the bag with the measuring cup, you likely packed in more than the recipe calls for. Always spoon flour into the measuring cup and level it off with a straight edge, or weigh it if you have a kitchen scale. Any one of these three things can push your bars from chewy to cakey, and all three together will do it every time.

Baking Pan Variations

9×9 Pan - The Standard (Recommended): This is the pan the recipe is written for. A 9×9 gives you thick, bakery-style bars with a fudgy center, slightly crisp edges, and a clean cut. Expect 16 generous squares or 25 smaller bite-sized pieces depending on how you slice them. Bake at 350°F for 22-25 minutes, pulling when the center looks just barely set.

9×13 Pan - For a Crowd: A 9×13 pan works well if you want thinner bars or need to feed a larger crowd. The batter will spread into a thinner layer, so reduce your baking time by 5-7 minutes and start checking for doneness around the 18-minute mark. The edges will set faster than the center, so watch the middle - it should look just barely set when you pull them.

8×8 Pan - Extra Thick: Want an even thicker, fudgier bar? Use an 8×8 pan with the standard recipe. The batter will be deeper in the pan, so add 3-5 minutes to the baking time and tent loosely with foil around the 20-minute mark if the top is browning too fast before the center is done.

Fun Seasonal Variations

One of the best things about this recipe is how effortlessly it transforms into a holiday dessert - no new recipe needed, no extra steps, just a simple M&M color swap. Here's how to make monster cookie bars work for every season of the year.

• 🎄 Christmas Monster Cookie Bars - red & green M&Ms
• 💝 Valentine's Day Monster Cookie Bars - red & pink M&Ms
• ☘️ St. Patrick's Day Monster Cookie Bars - all-green M&Ms
• 🐣 Easter Monster Cookie Bars - pastel mix M&Ms
• 🎃 Halloween Monster Cookie Bars - orange & black M&Ms

Want to adapt this dough for different pans and flavors? Check out my full guide, How to Make Cookie Bars (and Other Easy Dessert Bars), for pan sizes, bake times, and mix‑in ideas.

How to Store Monster Cookie Bars

Have leftovers? Use them to make my Brownie Stuffed Chocolate Chip Cookies - the best thing you can do with a leftover monster cookie bar!

• Room Temperature: Store in an airtight container, layered with parchment paper, for up to 5 days. Keep away from heat and direct sunlight - a pantry or cabinet works perfectly.
• Refrigerator: Refrigerate in an airtight container for up to 1 week. The bars will firm up when cold - let them sit at room temperature for 15-20 minutes before eating for that soft, chewy texture back.
• Freezer: Freeze for up to 3 months. Cut into individual bars, wrap each one in plastic wrap, and store in a zip-lock freezer bag. Thaw at room temperature for 30-45 minutes - they taste freshly baked.
• Freezing the unbaked dough: Press raw dough into the lined pan, wrap tightly, and freeze for up to 2 months. Thaw overnight in the fridge and bake as directed, adding 2-3 extra minutes.

More Cookie Bars Recipes

• Salted Caramel Pretzel Brownies
• Cheesecake Chocolate Brownies
• Low Carb Chocolate Brownies
• 2 Ingredient Lemon Bars