Heat oven to 350°F. Grease and flour 2 (9-inch) round cake pans; set aside.
Combine flour, baking powder, baking soda and salt in bowl; set aside.
Combine 1 1/2 cups sugar, 3/4 cup butter and 1 tablespoon orange zest in another bowl. Beat at medium speed, scraping bowl often, until creamy. Add 1 egg at a time, beating well after each addition until light and fluffy. Add flour mixture alternately with buttermilk, beating just until combined.
Spread batter evenly into prepared pans. Bake 27-32 minutes or until toothpick inserted in center comes out clean. Let stand 10 minutes; remove from pans. Cool completely.
Combine all filling ingredients in bowl. Beat at medium speed until fluffy. Reserve 1/2 cup filling.
Place 1 layer onto serving plate. Spread remaining filling over cake. Top with remaining cake layer. Cover; refrigerate while preparing frosting.
Combine reserved filling and all frosting ingredients in bowl. Beat at high speed, scraping bowl often, until stiff peaks form. Frost top and sides of cake.
Serve immediately or cover and refrigerate for up to 6 hours. Garnish with orange zest, if desired. Store refrigerated.
*Substitute 2 teaspoons vinegar or lemon juice plus enough milk to equal 3/4 cup. Let stand 5 minutes.