For Cake: Preheat oven to 375°F Grease and flour 2 8-inch round cake pans. Cream butter and sugar. Add orange peel. Sift together the flour, baking powder, baking soda and salt. Combine the water and orange juice. Add to creamed mixture, alternating with dry ingredients, beating well after each addition. Beat egg whites to stiff peaks. Fold into batter. Pour into prepared cake pans. Bake for 25-30 minutes or until cake tester inserted in center comes out clean. Remove from oven and cool for 5 minutes on racks. Remove cakes from pans and cool completely. When cool, spread with filling and frosting.
For Filling: In medium saucepan, combine sugar, cornstarch, orange rind and salt. Gradually blend in orange juice, water and egg yolks. Cook over medium heat, stirring constantly, until thick and bubbly. Remove from heat; stir in butter until melted. Cool before spreading between cake layers.
For Frosting: Cream the butter. Add the confectioner's sugar, orange rind, salt and 4 tablespoons of juice. Beat until spreading consistency, adding more juice if needed. Spread on sides and top of cake.