Wilton's recipe for a pure white buttercream icing is made from vegetable shortening, butter, milk and confectioner's sugar. Clear vanilla extract, which is sold by Wilton, adds flavor without the yellow or brown tint of traditional vanilla extract. The standard Wilton recipes yields 2 1/4 cup of buttercream frosting.
In a large mixing bowl, combine 1/2 cup of butter or margarine and 1/2 cup of solid vegetable shortening. Using an electric mixer, beat the mixture until it is light and airy. Add 1 teaspoon of vanilla extract to the mixture, and beat it again.
With the mixer on medium speed, slowly add 1 cup of sugar at a time, and regularly scrape the bottom and the sides of the bowl. Once the mixture is well combined, the icing appears dry. Slowly add 2 tablespoons of milk while continuing to beat the mixture at medium speed. To avoid drying out the icing, cover it with a damp cloth until use.
If the cake requires a thinner icing, add 2 tablespoons of milk, water or corn syrup. To make a stiffer icing, replace the butter in the first step with an additional 1/2 cup of shortening and 1/2 teaspoon of no-color butter flavor.