How to Make It
Heat oven to 350º F. Butter two 8- or 9-inch round cake pans and line the bottom of each with a round of parchment paper.
In a large bowl, whisk together the flour, granulated sugar, cocoa powder, brown sugar, baking soda, and salt.
In a small saucepan, combine the butter and 1 cup water and bring to a boil. Add to the flour mixture and, using an electric mixer, mix until combined. Beat in the eggs, one at a time, then the sour cream and vanilla.
Divide the batter evenly between the prepared pans. Bake until a toothpick inserted in the center of each cake comes out clean, 35 to 45 minutes. Cool in the pans for 20 minutes, then turn out onto a rack to cool completely.
Transfer one of the cakes to a platter and spread with ¾ cup of the frosting. Top with the remaining cake and spread with the remaining frosting.