Preheat oven to 350 degrees F. Line 9-inch baking pan with heavy-duty aluminum foil.
In a large bowl, combine butter and sugar. Beat at medium speed with an electric mixer until fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla extract.
In a small bowl, combine flour, cocoa, baking powder, and salt. Gradually add to butter mixture, beating until combined. Spoon batter into prepared pan.
Bake for 22 minutes or until a wooden pick inserted in center comes out clean. Spread chocolate frosting over hot cake. Let cool completely. Cut into squares and serve with whipped cream, if desired.
In a medium saucepan, melt butter over medium-low heat. Add cocoa, buttermilk and 1 cup confectioners' sugar. Stir in candy bars and allow to melt. Stir in remaining confectioners' sugar and immediately spread over hot cake.
Yield: 1 cup