Note: Be sure to use Dutch-process cocoa (a brand like Droste) for both the cake and the icing, instead of regular cocoa powder. It will significantly affect the flavor.
1 1/2 cups (3 sticks) unsalted butter, plus more for pan
1 1/4 cups unsweetened Dutch-process cocoa powder
1 1/4 cups hot water
3 cups all-purpose flour
1 1/4 teaspoons coarse salt
1 teaspoon baking powder
1 teaspoon baking soda
2 1/4 cups granulated sugar
4 large eggs
1 tablespoon plus 1 teaspoon pure vanilla extract
1 cup sour cream
. Preheat oven to 350 degrees. Butter two 9-by-2-inch round cake pans, line bottoms with parchment paper, and butter paper. Or just use nonstick spray oil. Whisk together cocoa powder and hot water until smooth.
. Sift together flour, salt, baking powder, and baking soda; set aside. Melt butter and sugar in a saucepan over medium-low heat. Remove from heat, and transfer to a mixer. Beat on medium-low speed until cooled, 4 to 5 minutes.
. Add eggs, one at a time, beating after each addition. Beat in vanilla and cocoa-powder mixture. Reduce speed to low. Add flour mixture in two batches, alternating with sour cream and beginning and ending with flour. Beat until just combined.
. Divide batter between pans, and bake until a cake tester inserted into centers comes out clean, about 35 minutes. Transfer pans to a wire rack to cool for 45 minutes. Invert cakes onto rack, peel off parchment, and let cool completely.
. Using a serrated knife, trim tops of cake layers to make level. Transfer one cake layer to a cake stand, and spread with 1 cup frosting. Top with remaining cake layer, and coat top and sides with remaining frosting, spreading it in a swirling motion.
BEST CHOCOLATE FROSTING
3 1/2 cups confectioners’ sugar
1 cup Dutch-process unsweetened cocoa powder
12 tablespoons (1 1/2 sticks) unsalted butter, room temperature
1/2 cup milk, room temperature
2 teaspoons pure vanilla extract
. In a small bowl, sift together the confectioners’ sugar and cocoa powder. Add butter, milk, and vanilla; stir until smooth and free of lumps.