Preheat oven to 350°F (180°C). Spray 2 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.

In a large bowl, whisk together brown sugar, melted butter, and peanut butter until combined and creamy. Add eggs and vanilla, whisking just until combined.

In a medium bowl, whisk together flour, baking powder, salt, and baking soda. Gradually add flour mixture to sugar mixture alternately with buttermilk, beginning and ending with flour mixture, whisking just until combined after each addition. Transfer one-third of batter (about 2 cups [454 grams]) to a medium bowl; gently fold in melted chocolate until combined. Using a 2-tablespoon spring-loaded scoops, alternately scoop peanut butter and chocolate batters into prepared pans. (Each pan will get about 2 cups [about 453 grams] peanut butter batter and about 1 cup [about 283 grams] chocolate batter). Using a wooden pick or cake tester, swirl batters together. Gently tap pans on counter 2 to 3 times.

Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks.

Place 1 cake layer on a cake stand. Spread 1½ cups (about 375 grams) Chocolate Cream Cheese Frosting on top. Top with remaining cake layer. Spread remaining frosting on top and sides of cake. Cover and refrigerate until ready to serve.