CAKE: Preheat the oven to 350°. Grease three 6"×2" cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans.

Whisk the flour, cocoa powder, espresso powder, baking powder, baking soda, and salt together in a large bowl until thoroughly combined. Set aside. In a medium bowl, whisk the eggs, granulated sugar, brown sugar, oil, and vanilla together until completely smooth. Pour half of the wet ingredients into the dry ingredients. Then half of the buttermilk. Gently whisk for a few seconds. Repeat with the remaining wet ingredients and buttermilk. Stir until combined; do not overmix. The batter will be thin.

Pour batter evenly into cake pans. Bake for around 18-21 minutes or until the cakes are baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling. DO AHEAD: Cake layers can be baked, cooled, and covered tightly at room temperature overnight.

FROSTING: With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add the peanut butter, confectioners’ sugar, heavy cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 full minutes. Add up to ¼ c more confectioners’ sugar if frosting is too thin or another Tbsp of cream if frosting is too thick.

ASSEMBLY: Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place 1 cake layer on your cake stand, cake turntable, or serving plate. Evenly cover the top with about ¾ c of frosting. Top with 2nd cake layer and evenly cover the top with about ¾ c of frosting. Top with the third cake layer. Spread the remaining frosting all over the top and sides. A bench scraper and small offset spatula are handy for decorating. Top with crushed Reese’s Pieces, if desired. DO AHEAD: Frosted cake or unfrosted cake layers can be frozen up to 2-3 months. Thaw overnight in the refrigerator and bring to room temperature before decorating/serving.

Store leftovers in the refrigerator for up to 3-4 days.