Grease three 9-inch round baking pans or three 8x8x2-inch square baking pans; lightly dust each pan with 1 teaspoon of the cocoa powder. In a medium bowl, stir together the remaining cocoa powder, flour, baking powder, and baking soda. Set aside.
Preheat oven to 350 degrees F. In a large bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds. Add sugar; beat until combined. Add eggs, one at a time, beating for 30 seconds after each addition.
Beat in chocolate and vanilla. Alternately add flour mixture and milk to chocolate mixture, beating on low speed until well mixed. (Batter will be thick.)
Divide batter among prepared pans; spread evenly. Bake in the preheated oven for 20 to 25 minutes or until a toothpick inserted near the centers comes out clean. Cool in pans on wire racks for 10 minutes. Remove from pans. Cool completely on wire racks.
Make chocolate curls, if using. Arrange curls in an even layer on a parchment or waxed paper-lined tray and set aside in a cool, dry place. To assemble cake, spread Chocolate-Sour Cream Frosting on tops of two of the layers; stack. Add top layer; frost the top and sides of the cake. If desired, while frosting is still moist, use a 4- to 6-inch wooden skewer or tweezers to lightly press chocolate curls into frosting around the bottom two-thirds of the cake. If desired, garnish top with raspberries and mint. Store, covered, in the refrigerator. Makes 12 to 16 servings.
Chocolate-Sour Cream Frosting
In a large saucepan melt chocolate pieces and butter over low heat, stirring frequently. Cool for 5 minutes. Stir in sour cream. Gradually add powdered sugar, beating until smooth. This frosts tops and sides of two 8- or 9-inch cake layers.