The butter cream icing used by the Disney Grand Floridian bakery, who do the majority of Disney wedding cakes, is VERY different from what most Americans are used to. You are correct as far as the butter and shortening, but no confectioners (powdered) sugar goes into the recipe. The butter used is a fine low water brand imported from Europe, and the sweet in the recipe comes from a product called "creme fondant". Think of the drizzle icing you see on many breakfast pastries or danish. All of these elements make a very smooth, non-gritty icing. It melts in your mouth and leaves no greasy after taste. The flavor is purposely a very subtle vanilla, so as not to compete with the chosen cake/filling choices. And it makes an excellent filling as well.
And a quick note about peanut butter filling, Disney doesn't use straight from the jar peanut butter for their filling, it's a combination of peanut butter and creme cheese filling. Firm like any cake filling, and even better than plain old peanut butter!
