Rocky Road Cupcakes Adapted from Cupcakes by Pamela Clark

Marble Cake 2 sticks butter softened 1 t. vanilla extract 1 1/3 c. granulated sugar 4 eggs 2/3 cup milk (I substitute with 1/3 c. sour crm. and 1/3 c. milk) 2 1/4 c. flour 1 t. baking powder 1 t. baking soda 1/2 t. salt 3 T. cocoa powder 4 t. milk, e...