preheat oven to 350

prepare an 8" round or square baking pan with non-stick spray

combine the flour, sugars, cocoa and baking powder in a bowl and set aside

combine the Dr. Pepper and chocolate chips in a saucepan. heat on low, just until the chips are melted. cool slightly and set aside

combine the egg, buttermilk, oil and vanilla in a LARGE bowl

slowly pour the Dr. Pepper mixture into the egg-buttermilk mixture

gradually add the dry ingredients. after all the dry mixture is added, beat another 2 minutes

pour the batter into the prepared pan

bake 30 minutes or until the center is done

cool and remove from the pan before frosting

heat the Dr. Pepper, margarine and cocoa. stir in the powdered sugar and vanilla

allow to cool slightly and either pour over the cake or spread the frosting evenly on the cake