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, baking soda, baking powder, salt, and black cocoa powder.
In a large bowl, whisk together the granulated sugar, brown sugar, buttermilk, vegetable oil, eggs, vanilla extract, and apple cider vinegar until smooth.
Gradually add the dry ingredients to the wet ingredients and mix just until combined.
Slowly stir in the hot coffee until the batter is smooth.
Divide the batter evenly among the prepared cake pans.
Bake for 22 to 26 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool the cakes in the pans for 10 minutes before transferring them to wire racks to cool completely.
To prepare the buttercream, beat the butter for 10 to 15 minutes until light and fluffy.
Gradually beat in the powdered sugar and black cocoa powder.
Mix in the vanilla extract and salt.
Add the heavy cream, one tablespoon at a time, until the frosting is smooth and spreadable.
Beat for 3 to 5 minutes until light and fluffy.
Place one cake layer on a cake plate.
Spread about 1 cup of buttercream evenly over the top.
Repeat with the second cake layer and another cup of buttercream.
Place the third cake layer on top.
Apply a thin crumb coat over the entire cake and refrigerate for 20 minutes.
Frost the top and sides with the remaining buttercream. You can use a piping bag and tip to pipe on the remaining frosting.
Create swirls or smooth the frosting as desired.
Top with fresh blackberries if desired.