Grand Marnier Cream butter; gradually add sugar and beat until light. Add fruit rinds; add eggs one at a time, beating thoroughly after each. Add sifted dry ingredients alternately with buttermilk, beating after each addition until smooth. Fold in nuts. Pour into greased 9 or 10-inch 2-quart tube pan. Bake at 350 degrees for 1 hour. Strain fruit juices into saucepan. Add sugar and rum; bring to boil. Pour slowly over hot cake in pan. If all liquid is not absorbed, pour remainder on later. FOR ICING: Bring butter to room temperature. Mix all ingredients, adding enough Grand Marnier to make a spreading consistency. Spread on cooled cake.