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