1. Adjust oven rack to top of lower third of oven. Heat oven to 350 degrees F. Grease two 8-inch square metal layer-cake pans. Line bottoms with waxed paper; grease paper; flour pans and paper.
2. Place sugar in small, heavy-duty plastic food-storage bag; seal; freeze 20 minutes. Place large mixing bowl and beaters in freezer 5 minutes.
3. Combine buttermilk and grated orange rind in a bowl. Sift together flour, baking powder, baking soda and salt onto waxed paper 3 times.
4. To cream, beat butter in chilled bowl with chilled beaters 3 minutes, until light in color. With mixer running, add chilled sugar in steady stream. Beat 3 to 4 minutes, until fluffy. Add eggs, one at a time, beating 30 seconds after each; then beat 1 to 2 minutes, until light and airy. Fold in half of flour mixture; fold in half of buttermilk mixture. Repeat with remaining flour and buttermilk mixtures. Pour into prepared pans.
5. Bake in 350 degrees F oven 20 to 25 minutes, until centers spring back when pressed with fingertip. Cool in pans on racks 10 minutes. Turn out onto racks to cool. Peel off paper.
6. Meanwhile, prepare frosting.
7. Place one layer on serving plate. Spread top and sides with half of frosting. Top with remaining cake layer. Frost. Refrigerate 1 hour before serving.
1. Beat butter, milk, orange juice, orange rind, salt and 1 cup confectioners sugar in bowl until smooth. Slowly beat in another 3 cups confectioners sugar; continue beating until spreading consistency.