In a large bowl, beat the shortening and butter with an electric mixer on a medium speed 2-3 minutes or until well combined.
Gradually add the powdered sugar, one cup at a time, beating well between each addition, until the frosting appears smooth. When all the sugar has been mixed in, the frosting will appear dry.
Add the milk, peppermint extract, salt, and vanilla. Beat at medium speed 3-4 minutes or until light and fluffy.