Grease and lightly flour three 8x1 1/2 inch round baking pans. Stir together flour, baking powder, and baking soda. Set pans and flour mixture aside.
Beat butter with an electric mixer on medium to high speed 30 seconds. Add sugar, vanilla, and almond extract to butter and beat until fluffy. Alternately add flour mixture and buttermilk, beating on low to medium speed just until combined.
Thoroughly wash beaters. In a medium mixing bowl beat egg whites until stiff peaks form (tips stand straight). Gently fold beaten egg whites into batter. Fold in the 1 1/2 cups pistachio nuts and the orange peel.
Pour batter into prepared pans. Bake in a 350 oven for 30-35 minutes or until a wooden toothpick inserted near the center of each cake comes out clean. Cool in pans on wire racks 10 minutes. Remove cakes from pans and completely cool on wire racks. Frost with White Chocolate Buttercream. Press remaining pistachio nuts on sides of cake. Cover and store cake in refrigerator for up to 3 days. Let stand at room temperature for 30 minutes before serving. Makes 12 servings.
WHITE CHOCOLATE BUTTERCREAM: In a medium mixing bowl combine 1/2 cup granulated sugar and 2 Tablespoons all-purpose flour; add the 6 reserved egg yolks. Beat mixture with a wire whisk until combined; set aside. In a heavy, medium saucepan heat 1 1/2 cups milk over medium heat just to boiling. Remove from heat. Gradually beat hot milk into egg mixture with the wire whisk; return entire mixture to saucepan. Cook over medium heat until bubbly, whisking constantly. Cook for 2 minutes more. Remove from heat. Add 6 ounces chopped white baking bar, 1 1/2 teaspoons vanilla, and 1/2 teaspoon almond extract. Let stand 1 minute; stir until smooth. Transfer mixture to a bowl. Cover surface with plastic wrap to prevent a skin from forming; cool to room temperature. In a medium mixing bowl beat 1 cup softened butter (no substitutes) on medium to high speed until fluffy. Add cooled baking bar mixture, one fourth at a time, beating on low speed after each addition until blended.