Grandmother's potato cake

1 cup butter or maragine
 2 cups sugar
 2 eggs
 1 cup cold mashed potatos
 1 tsp. vanilla extract
 2 cups all-purpose flour
 1/4 cup baking cocoa
 1 tsp. baking soda
 1 cup milk
 1 cup chopped nuts
 In mixing bowl,cream butter and sugar until fluffy. Add eggs,one at a 
 time,beating well after each addition. Blend in potatoes and vanilla. 
 Combine flour,cocoa,and baking soda;add alternately with milk,blending 
 well after each addition. Stir in nuts. Pour into a greased 13-in. x 
 9-in.x 2-in. baking pan. Bake at 350 degrees for 40-45 minutes or until 
 cake tests done. Cool on a wire rack.