Preheat oven to 325°F (170°C). Spray 3 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.

In a large bowl, whisk together flour, brown sugar, baking powder, salt, and baking soda.

In a medium bowl, place cocoa; whisk in hot coffee or hot water until cocoa dissolves and mixture is smooth. Whisk in sour cream, oil, and vanilla until combined. Whisk in eggs. Add cocoa mixture to flour mixture, and whisk until no dry streaks remain. Divide batter among prepared pans (about 2½ cups or 678 grams per pan).

Bake until a wooden pick inserted in center comes out with a few moist crumbs, 40 to 45 minutes. Let cool in pans for 10 minutes. Invert cake layers onto wire racks, and let cool completely.

Using a large serrated knife, level cooled cake layers if desired.

Place 1 cake layer on a serving plate. Spread 1½ cups (about 300 grams) Black Cocoa Buttercream onto cake layer. Top with second cake layer, and spread 1½ cups (about 300 grams) buttercream on top. Top with remaining cake layer. Spread remaining buttercream on top and sides of cake as desired. Cover and refrigerate for up to 3 days.