How to Make It
Preheat oven to 350°.
Beat egg whites and cream of tartar at high speed of a mixer until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time; beat until stiff peaks form. Set aside.
Beat 1 1/4 cups sugar and margarine in a large bowl at medium speed of a mixer until well-blended (about 5 minutes). Add mashed potatoes and egg; beat well. Combine flour and next 7 ingredients (flour through nutmeg). Add flour mixture to potato mixture alternately with milk, beginning and ending with flour mixture. Fold egg white mixture and vanilla into batter; pour batter into a 12-cup Bundt pan coated with cooking spray.
Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes; remove from pan. Cool completely on a wire rack. Cut into 12 slices. Sprinkle with cocoa; dollop each serving with 1 tablespoon whipped topping.