This frosting will not crust is because it does not have a high enough sugar to fat ratio.
FROSTING, ICING, ETC RECIPE HELP

INGREDIENTS
1/2 cup high-ratio shortening or preferably a non-trans fat free shortening
1/2 cup (1 stick) unsalted butter or shortening*

4 1/2 cups sifted powdered sugar; sift onto waxed paper and then, measure
2 tablespoons (1/8) to 4 tablespoons (1/4 cup) heavy whipping cream or whole or 2% milk
2 teaspoons vanilla extract
1/4 teaspoon salt
1 – 2 teaspoons light corn syrup, optional
Extra heavy cream to thin or extra sifted powdered sugar to thicken, if necessary

NOTE: Corn syrup will help make the icing shiny. *If you use all shortening in the recipe, you may find you need 4 tablespoons (1/4 cup) heavy cream in the recipe.
For use as a frosting dam, use only 2 tablespoons heavy cream and do not add in corn syrup. Make sure frosting does not crack when piping. If it does, the frosting it too thick and dry. Thin slightly with drops of cream or milk and beat on low until combined.


INSTRUCTIONS
1. Fit a stand mixer with a paddle attachment. Beat shortening on low until softened and smooth. Add butter and continue beating on low until both are softened, smooth and combined. Scrape the side and bottom of the bowl with a large rubber spatula.
SARAH SAYS: I like to first beat the shortening until softened, and then add the butter. I find I get a better consistency when I do.

2. Add 1 cup of powdered sugar at a time beating on low until well mixed before adding more. Repeat until all of the sugar has been added. The frosting will look very dry and crumbly. Scrape the side and bottom of the bowl with a large rubber spatula.

3. Add cream, vanilla and salt. Increase the mixer speed to medium-low for about 30 seconds, and then return the mixer speed to low for about 1 minute. When you remove the mixing bowl from the mixer, give the icing a few turns with a large rubber spatula, making sure you scrape the sie and bottom of the bowl.

Add additional cream to thin to the required consistency or more powdered sugar to stiffen. For thinner frosting, add in 1 tablespoon corn syrup in addition to 2 – 3 tablespoons heavy cream called for in the recipe. Mix well on low speed until incorporated. The more you beat the frosting, the fluffier the consistency becomes.

Keep bowl covered with a damp cloth while using so the icing does not dry out.

STORAGE
Cakes iced with this frosting do not need refrigeration if it remains on the cooler side of room temperature and you do not use more than 1/4 cup (4 tablespoons) cream in the above recipe.

Frosting is best stored in the refrigerator for about 1 week for maximum freshness. Can be frozen for up to one to two months.

TINTING
Frosting can be tinted. If you desire pure white, use all shortening (substitute the butter with shortening), and use clear vanilla extract instead of pure vanilla extract.