Orange Chiffon Icebox Cake

2 1/2 cups sifted cake flour
1 tablespoon baking powder
1 teaspoon salt
1 1/3 cups sugar
1/2 cup vegetable oil
5 large eggs, separated
3/4 cup orange juice
3 tablespoons orange zest
1/2 teaspoon cream of tartar
Orange Icebox Filling
Orange Buttercream Frosting

Combine flour, baking powder, salt and sugar in a large mixing bowl.  Make a well in the center and add vegetable oil, egg yolks and orange juice.  Beat at medium high speed with an electric mixer until smooth.  Stir in the orange zest.  In a mixer bowl, beat egg whites and cream of tartar at medium-high speed  until stiff peaks form..  Gently fold the egg whites into the flour mixture.  Divide batter evenly into 3 greased and floured 9-inch round cake pans.  Bake at 350 degrees for 18-20 minutes or until a wooden pick comes out clean.  Cool in pans on a wire rack for about 10 minutes.  remove from pans and cool completely on wire racks.

Orange Icebox Filling

3/4 cup milk
1/4 cup cornstarch
1- 14oz. can sweetened condensed milk
3 large eggs
1/2 cup thawed frozen orange juice concentrate
3 tablespoons butter
1 tablespoon orange zest

Whisk together 3/4 cup milk and 1/4 cup cornstarch in a 3 quart heavy saucepan, whisking until cornstarch dissolves.  Whisk in the sweetened condensed milk and eggs until blended; whisk in orange juice concentrate.  Bring to a boil over medium heat, whisking constantly.  Boil 1 minute, whisking constantly, or until mixture thickens.  Remove from heat; whisk in butter and orange zest until smooth.  Pour filling into a bowl; place in a larger bowl filled with ice and stir until mixture is cold.

Orange Buttercream Frosting

1/2 cup butter, softened
1 pound package powdered sugar
1/3 cup milk
2 tablespoons orange zest

Beat the butter at medium speed with an electric mixer until creamy; gradually add about 1 cup of the powdered sugar and beat at low speed until blended.  Add the milk,  Gradually add the remaining  powdered sugar, beating until blended.  Stir in the orange zest.

To assemble the cake:

Spread the Orange Icebox filling on top of each of the 3 layers.  Chill in the refrigerator for at least 4 hours.  Spread the buttercream frosting evenly on the sides of cake.  Keep in refrigerator until ready to serve.  Garnish with orange slices if desired.