Special equipment: cake boards for each tier
Preheat the oven to 300 degrees F. Grease and flour one of each: 12-by-2-inch, 9-by-2-inch and 6-by-2-inch round cake pans.
Using a heavy-duty mixer, cream 3 sticks of the margarine and 1 pound of the confectioners' sugar until the mixture is very light and fluffy, about 5 minutes. One at a time, break 6 of the eggs into the batter, beating only as long as it takes to break another egg between additions. Scrape down the sides of the mixing bowl.
Measure 1 pound of the sifted cake flour into an empty confectioners' sugar box, filling to the top. Add the flour to the sugar and egg mixture, stirring gently on low speed. Scrape the sides of the mixing bowl. Add 1 teaspoon of the vanilla extract and mix again. Transfer the batter to a bowl and reserve. Repeat the steps with the other half of the ingredients.
Pour the two batches of batter into the prepared pans and bake until the center is set, about 1 hour. Check for doneness by touching the surface lightly with your fingertip or by inserting a toothpick in the center; it should come out clean.
Cool the cakes in the pans for 10 minutes, and then run a thin knife or spatula between the cake and the pan to loosen the edges. Invert the cakes onto wire racks. Place on cake boards to frost with Buttercream Frosting. Slide the layers off the cake boards to stack and decorate with White Cream Decorator Frosting.
With an electric mixer, cream the butter and add the sugar slowly, beating together until smooth. Add the cream by the tablespoon, beating well after each addition and adding just enough liquid to make a smooth, spreadable icing, Add the vanilla extract and continue to beat the icing on high speed until it is very light and fluffy about 5 minutes.
Using an electric mixer, cream the shortening and add the sugar gradually, blending well, and beat until fluffy. Beat in the salt, and then add 4 to 5 tablespoons water by the tablespoon, beating continuously and adding just enough to achieve a smooth, spreadable consistency.
Add the vanilla extract and beat on high until very fluffy. This frosting can be colored with food coloring if desired and transferred to pastry bags fitted with decorator tips for decorating.
You will need to make this recipe in two batches to fit in a standard stand mixer. Using solid white shortening instead of butter as well as clear vanilla extract results in a pure white frosting that you can tint any color you like. If you can't find clear vanilla extract, you can use regular vanilla extract, but your icing won't be white. The regular extract will add a cream color to the icing.
Recipe adapted from Georgia Cooking in an Oklahoma Kitchen by Trisha Yearwood(c) Clarkson Potter 2008