Preheat oven to 325F.

Coat a 10-inch tube pan or 12-cup Bundt pan with cooking spray. Combine 1 tablespoon granulated sugar and 1 tablespoon flour. Sprinkle over oiled pan, tapping out excess.

Beat butter with a mixer at high speed until light and fluffy. Gradually add 1 3/4 cups granulated sugar. Continue beating until well combined. Beat in eggs, one at a time. Sift 3 cups flour, baking soda and salt into a medium bowl.

Combine buttermilk, orange juice and lemon juice. Add flour mixture and buttermilk mixture alternately to butter mixture, beginning and ending with flour mixture; beat at low speed. Stir in the first amounts orange and lemon rinds. Pour batter into prepared pan.

Bake 60 to 70 minutes, until top is golden brown and a wooden pick inserted in the center comes out clean.

To prepare icing, combine butter and powdered sugar using a mixer at medium speed. Add second amounts of orange and lemon rinds, and juices, 1 tablespoon at a time. Beat well. Cool cake in pan on a wire rack 10 minutes. Turn onto a serving plate. Pour one-third of icing over cake while still warm. Cool completely and spread with remaining icing. Garnish with orange zest, if desired.