This fast recipe could easily be made with regular chocolate box mix, but I like to make it for my friends with celiac using gluten-free mix. It takes all the guess work out of converting your favorite chocolate cake recipe to use gluten-free flour. Since cocoa powder is GF, it's is the way to go if you need a GF option – use it to flour the pans instead of all-purpose flour.
three 6-inch cake pans; one 6-inch cardboard cake round; plate, cardboard cake drum or rotating cake stand
For the cake: Preheat the oven to 350 degrees F. Spray three 6-inch cake pans with cooking spray. Line each pan with parchment paper rounds. Spray the parchment paper and dust cocoa powder over the pan and paper. Tap the pan over a sink or trash can to get rid of excess powder.
Whisk together the cake mix, pudding mix, water, oil and sour cream in large bowl. Add the eggs one at a time, whisking until full incorporated after each addition. Toss the chocolate chips with the cocoa powder in a small bowl. Fold the chocolate chips into the batter.
Divide the batter evenly among the prepared pans. Bake until a toothpick inserted in the center comes out clean, about 20 minutes. Allow to cool in the pans on a wire rack for 10 minutes, then turn the cakes over onto the wire rack to cool completely.
For the chocolate buttercream frosting: Add the butter to the bowl of a stand mixer fitted with a paddle attachment and stir on low speed until combined. Gradually add the confectioners' sugar alternating with the milk until incorporated. Gradually add the cocoa powder, salt and melted chocolate and mix until combined. Add to a large piping bag and tie off with a rubber band.
Place the first cake layer on a 6-inch cardboard round and place on a rotating cake stand. Pipe a layer of about 1/2 cup buttercream on top and use an offset spatula to spread into an even layer. Place the second cake layer on top of the buttercream and pipe another 1/2 cup buttercream on top. Spread into an even layer, then top with the third cake layer.
To apply a crumb coat, pipe about 2 cups buttercream on the top and sides of the cake and use an offset spatula or bench scraper to make smooth. Remove excess buttercream and scrape into a bowl. Transfer the cake to the refrigerator and chill for 30 minutes.
Remove the chilled cake from the refrigerator. Pipe about 2 tablespoons buttercream onto the center of a 12-inch cake drum or plate; this will help hold the cake in place while you frost. Place the cake drum on a rotating cake stand. Place the cake on top of the buttercream.
Pipe 2 to 3 cups buttercream up the sides and top of your cake, smoothing the edges with a bench scraper. Scrape excess buttercream into a bowl to keep the bench scraper clean.