• Preheat oven to 350℉. Grease two 8 inch round cake pans. We also added circles of parchment to the bottom of each pan (as usual).
• This recipe can be prepared in a large mixing bowl, just as when making brownies from a mix. No mixer needed.
• Combine dry ingredients in large bowl and whisk together until combined. (This is the brownie mix, flour, sugar, cocoa powder, and baking powder.)
• Add the remaining ingredients (eggs, sour cream, fudge packet (if your mix has one), water, and vegetable oil) and stir until well combined. I find it easiest to use a rubber spatula for this step.
• Divide the cake batter between two 8 inch cake pans. (This recipe makes about 5 cups of batter.)
• Bake at 350℉ for 25-30 minutes or until a toothpick can be inserted into the cake layers and comes out clean or with just a few crumbs attached.
• Cool the cake layers in the cake pans (preferably on a cooling rack) for 5 minutes before turning out. Cool completely before frosting.

For the Chocolate Buttercream

• Add butter and mix on low to medium speed until smooth. Add vanilla and blend into the butter.
• Add powdered sugar, salt, and cocoa powder.
• Add most of the milk and mix on low speed, increasing to medium as the ingredients become incorporated.
• Continue to mix adding remaining milk as needed. It may take 3-5 minutes to reach the smoothness and consistency you like. It will take longer if using a hand mixer. As you mix, the frosting will become very smooth. You can add additional milk (1 Tablespoon at a time) if the frosting is too thick or additional powdered sugar if too soft.

Decorating the Cake

• Place the first cake layer on a cake pedestal and spread with frosting. Top with the second cake layer and frost the cake around the sides and top with a thin layer of frosting. (This is the "crumb coat".)
• At this point, we like to chill the cake in the freezer for 15 minutes (or longer in the refrigerator) to firm everything up. (Chill the bowl of frosting in the refrigerator during this time if needed).
• Frost the cake with a second layer of frosting. Decorate however you like! At this point, we added texture around the sides of the cake using a small offset spatula, making diagonal strokes from bottom to top.
• We added additional piping on top of the cake, piping large frosting shells using a large star (1M) piping tip.
• Keep the cake in an airtight container or under a cake dome in a cool room in your house for 1-2 days or in refrigerator for longer. If refrigerating, you should remove the cake a couple of hours before serving so that it has time to warm and soften.