Preheat the oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.

In a medium bowl, whisk together the flour, salt, baking powder, and baking soda until evenly combined.

Place the blackberries in a small bowl and toss them with the tablespoon of flour. This helps suspend the fruit throughout the batter while baking.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed for 3 to 4 minutes, until very pale, light, and fluffy.

Add the eggs one at a time, beating well after each addition. Add the egg whites, followed by the vanilla extract, and mix until fully incorporated.

Reduce the mixer to low speed. Add the dry ingredients in three additions, alternating with the buttermilk, beginning and ending with the flour mixture. Mix only until each addition is just incorporated. The batter may look slightly curdled after adding the buttermilk — this is normal and will come back together once the final addition of flour is mixed in.

Using a rubber spatula, gently fold the floured blackberries into the batter until evenly distributed.

Divide the batter evenly between the prepared cake pans and smooth the tops.

Bake for 28 to 34 minutes, or until the tops spring back lightly when touched and a toothpick inserted into the center comes out clean.

Leave the cakes to cool in the pans for 15 minutes before turning them out onto wire racks to cool completely.

Reduce the mixer to low speed and gradually add the powdered sugar. Once incorporated, add the blackberry jam, vanilla extract, salt, and 1 tablespoon of heavy cream.

Increase the speed to medium-high and beat for another 3 to 4 minutes, until the buttercream is light, smooth, and fluffy.

If needed, add the remaining heavy cream a teaspoon at a time until the buttercream reaches a smooth, spreadable consistency.

Mix on the lowest speed for 1 to 2 minutes to remove any large air bubbles.

Spoon a thin layer of seedless blackberry jam over the buttercream, keeping it inside the buttercream border.

Repeat with the second cake layer.

Place the final cake layer on top, upside down, for the flattest finish.

Apply a thin crumb coat of buttercream over the entire cake and refrigerate for 20 to 30 minutes.

Cover the cake with the remaining buttercream and smooth the sides with an offset spatula or bench scraper.

Dot a few small spoonfuls of blackberry jam randomly around the sides of the cake and gently smooth with a bench scraper to create soft streaks. One or two passes is all you need — avoid overworking the jam.

Decorate the top with fresh blackberries and edible flowers.

Slice, serve, and enjoy.