This yields icing for one 2- or 3-layer 9-inch cake, or 2 dozen cupcakes
1 1/2 cups (3 sticks) unsalted butter, softened
2 tablespoons milk
9 ounces semisweet chocolate, melted and cooled to lukewarm
1 1/2 teaspoons vanilla extract
3 cups (or slightly more) confectioners' sugar
In a large bowl, beat the butter until creamy, about 2 to 3 minutes. Add the milk slowly and blend until smooth. Add the melted chocolate and beat well. Add the vanilla and beat another minute or so. Gradually add the sugar and beat until of desired consistency.
Use and store the icing at room temperature, as the icing will set if chilled. It can be stored in an airtight container for up to 3 days.
Preheat oven to 350 degrees F. Grease and lightly flour two or three (9x2-inch) round cake pans, then line the bottoms with wax paper.
In a medium bowl, sift together the flour, baking soda, and salt. Set aside.
In a large bowl, cream the butter and the sugars on the medium speed of an electric mixer until fluffy, about 2 to 3 minutes. Add the eggs one at a time, beating well after each addition. Add the chocolate, mixing until well incorporated.
Add the dry ingredients in thirds, alternating with the buttermilk and vanilla, beating after each addition until smooth. Stir in the sour cream. Divide the batter between the prepared pans.
Bake for 25 to 30 minutes, or until a cake tester inserted into the center of the cake comes out clean. Let the cakes cool in the pans for 10 minutes, Remove from the pans and cool completely on a wire rack.
When the cake has cooled, frost between the layers, then the top and sides of the cake. Frost with Chocolate Buttercream Icing.
Line two 12-cup muffin tins with cupcake papers. Spoon the batter into the cups to about two-thirds full. Bake until the tops spring back when lightly touched, about 20 to 22 minutes. Let the cupcakes cool in the pans for 10 minutes. Remove the cupcakes from the pans and cool completely on a rack before icing.
Makes one 2 or 3 layer 9-inch cake and serves 8 to 10, or approximately 2 dozen cupcakes