How to Make It
Melt butter in a heavy saucepan over medium-high heat. Stir in brown sugar, corn syrup, and cream of tartar; bring to a boil. Boil 5 minutes. (Do not stir.) Remove from heat; stir in powdered sugar, whipping cream, and vanilla.
Beat at medium speed with an electric mixer 2 minutes. Spread on cupcakes immediately.