There are a few versions of the Wilton recipe out there, but this is the one recommended in my class. I will probably stick with a pretty traditional Wilton recipe for "decorating," but there are plenty of other icing recipes that you could use as the base frosting on your cake. The Wilton recipes are meant to be stiff enough to hold the shape of your piping and decorations without "melting," which is why they don't use straight butter.
Class Buttercream Icing
stiff consistency, yield ~3 cups
source: Wilton Class I Student Guide
1 c. solid white vegetable shortening (For the batch I make as the base frosting on my cake, I use 1/2 c. vegetable shortening and 1/2 c. butter)
1 tsp. flavoring (clear vanilla abstract, or almond or butter abstract will work too)
2 Tbsp. milk or water
1 lb. confectioners' sugar (approx 4 c., sifted is best)
1 Tbsp. meringue powder
a pinch of salt
1. Cream shortening, flavor, and water.
2. Add dry ingredients and mix on medium speed until all incorporated. Blend an additional minute or so, until creamy.
*To alter to medium consistency, add 1 tsp. water to each cup of stiff consistency icing. To alter to thin consistency, add 2 tsp. water to each cup of stiff consistency icing.
** Chocolate variation: Add 3/4 c. cocoa or three 1 oz. unsweetened choc squares (melted), and an additional 1 to 2 Tbsp. water to recipe. Mix until well blended.