Cake: Place oven racks in top third and bottom third of oven. Heat oven to 375 degrees F. Grease three straight-sided 9 x 1-1/2-inch-round cake pans. Line 4 standard-size cupcake cups with paper liners.

Combine flour, baking powder, baking soda and salt in small bowl.

Beat together butter and brown sugar in large bowl on low speed until combined. Increase speed to medium and beat until creamy. Add eggs, one at a time, beating well after each addition. Beat in chocolate.

Add flour mixture alternately with buttermilk to egg mixture, beating after each addition, beginning and ending with flour mixture. Beat in boiling water and vanilla until smooth and well blended (batter will be thin).

Using dry measures, pour 3-2/3 cups batter into each prepared round cake pan. Divide remaining batter equally among lined cupcake cups.

Bake in 375 degree F oven, 20 minutes for cupcakes and 30 to 35 minutes for cake layers, or until toothpick inserted in centers comes out clean. Let cupcakes cool in pans 5 minutes; unmold onto wire rack and let cool completely. Cool layers in pans on wire racks 15 minutes. Invert onto wire racks and cool completely.

Frosting: Melt chocolate chips in small heavy saucepan over low heat until smooth. Let cool 15 minutes.

Beat butter and chocolate in large bowl until creamy. Blend in sour cream, vanilla and salt. Gradually beat in confectioners' sugar until smooth, creamy and spreadable.

Tear cupcakes into pieces. Coarsely grind in food processor.

Place 1 chocolate cake layer onto serving platter. Spread top with 1 cup frosting. Add second layer; spread with 1 cup frosting. Top with third layer. Spread thin layer of frosting around sides and top of cake. Reserve 1 cup frosting for top. Makes 16 servings.