*** Cake *** Cream butter in a large bowl until fluffy, gradually adding the sugar. Beat until light. Stir in rinds, add eggs one at a time beating after each. Combine flour, baking powder, baking soda and salt. Add alternately with buttermilk, beginning and ending with dry ingredients. Beat well after each addition. Fold in nuts and pour in a greased and floured 9-inch tube pan. Bake in a preheated 350oF oven for 50-60 minutes until test done. Cool in pan on a rack for 10 minutes. *** Hot Rum Sauce *** Boil sugar and water for 1 minute, cool slightly, add remaining ingredients. Carefully spoon over warm cake until all is absorbed. Let cool and dust with powdered sugar. Cover loosely to store.