Step 1

Preheat oven to 325°. Beat butter at medium speed with an electric stand mixer until creamy. Gradually add sugar, beating until light and fluffy. Add orange zest and vanilla; beat until blended. Combine flour and next 2 ingredients; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition.

Step 2

Beat egg whites at high speed until stiff peaks form. Stir about one-third of egg whites into batter; fold in remaining egg whites in 2 batches. Spoon into 3 greased (with shortening) and floured 9-inch round cake pans.

Step 3

Bake at 325° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely (about 1 hour). Frost as desired.

Step 4

Note: Cake layers may be baked in 3 (8-inch) square disposable aluminum foil pans. Increase bake time to 26 to 30 minutes.