Using a mixer, cream together shortening and butter, then add vanilla. Gradually add sugar in, one cup at a time (will be crumbly). Then add milk and beat at medium speed until light and fluffy.
Can be kept refrigerated in an airtight container, and stored for 2 weeks (re-whip before using).
Notes
Variation:For thin (spreading) consistency icing, add an additional 2 Tbsp light corn syrup, water or milk. *can replace shortening with more butter, it will just be a richer tasting frosting.