In a 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 x 9 x 2 inch baking pan. Bake at 350 F for 40-45 minutes or until cake tests done. Cool on wire rack. Yield: 12-16