Preheat oven to 350 degrees F.
Use 1 tablespoon of butter to grease 3 (9-inch) cake pans. Line the bottom of each pan with a parchment paper round.
Combine cocoa and chocolate in medium bowl. Pour boiling water over the chocolate mixture and whisk until smooth; reserve. Sift together the flours, baking soda, and salt into a medium bowl; reserve.
Place the 3/4 pound butter in large bowl and beat with electric mixer at medium-high until creamy, about 1 minute. Add dark brown and granulated sugars and beat on high until light and fluffy, about 3 minutes. Stop mixer and scrape down bowl with rubber spatula. Add the eggs, 1 at a time, beating 30 seconds on medium-high after each addition. Reduce speed to medium and add sour cream and vanilla. Beat until combined. Stop mixer and scrape down bowl. With mixer on low add about 1/3 of flour mixture, followed by about 1/2 of chocolate mixture. Repeat ending with flour mixture, beating until just combined. Stop mixer and scrape down sides of bowl. Stir gently to thoroughly combine. Divide batter evenly among cake pans and smooth with spatula. Bake until skewer inserted in center comes out clean, about 25 minutes. Cool the cakes in their pans on wire racks for about 15 minutes. Invert cakes, 1 at a time, onto a large plate, peel off parchment, and invert onto rack.
Cool completely before icing between layers, on sides, and on top with Vanilla Buttercream Icing. Serve at room temperature. (Any buttercream will not fare well in too warm conditions.)
9 large egg yolks
1 cup sugar
1/2 cup plus 2 tablespoons light corn syrup
1 1/4 pounds (5 sticks) unsalted butter, softened
2 teaspoons pure vanilla extract
In a large bowl beat the yolks with an electric mixer until light in color; reserve.
Combine the sugar and corn syrup in a small saucepan (preferably nonstick) over medium heat, stirring constantly, until the sugar dissolves and syrup comes to a rolling boil. Transfer syrup to a heatproof bowl to stop cooking. Pour a small amount of the syrup into the yolks and begin beating with electric mixer. Pour the remaining corn syrup into the mixture in a steady stream. Continue beating until completely cool and fluffy, about 5 minutes. Gradually beat in the butter and vanilla until the icing is light and fluffy, about 3 minutes. Use immediately. (Or can be covered and refrigerated up to 5 days.)
Yield: about 6 cups
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