1. Preheat oven to 350°. Combine first 4 ingredients in bowl of a heavy-duty electric stand mixer. Make a well in center of flour mixture; add oil, egg yolks, and orange juice. Beat at medium-high speed 3 to 4 minutes or until smooth. Stir in zest.
2. Beat egg whites and cream of tartar at medium-high speed until stiff peaks form. Gently fold into flour mixture. Spoon batter into 3 greased and floured 9-inch round cake pans.
3. Bake at 350° for 17 to 20 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).
4. Spread Lemon-Orange Buttercream Frosting between layers and on top and sides of cake.