Mix the sugar, flour and baking powder in a bowl. Bring the butter, baking cocoa and Coca-Cola to a boil in a medium saucepan, stirring to blend well. Pour over the dry ingredients gradually, mixing well.
Combine the buttermilk, egg and vanilla in a bowl; mix well. Add to the batter; mix well. Spoon into a greased and floured 8x8-inch cake pan.
Bake at 350 degrees for 25 to 30 minutes or until a cake tester comes out clean.
Prepare the Frosting»