Preheat oven to 375°.
Beat sugar and margarine at medium speed of a mixer until well-blended (about 5 minutes). Add the eggs and egg white, 1 at a time, beating well after each addition. Lightly spoon the flour into dry measuring cups, and level with a knife. Combine flour and the next 6 ingredients (flour through salt). Combine buttermilk, rum, orange rind, and vanilla. Add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Pour batter into a 10-inch tube pan coated with cooking spray. Bake at 375° for 50 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. Serve with Ambrosia.