In a mixing bowl, cream the butter until fluffy. THIS IS SO IMPORTANT! For good buttercream, you need the butter to be room temperature and beat it until its really fluffy, about 2 minutes or so.
Beat in the vanilla, butter flavor, salt, and butterscotch filling until just combined.
Add in the powdered sugar and beat until the frosting is light and fluffy. You want it to be able to form stiff peaks. If it becomes too thick, add in a very small splash of heavy cream mixing until it’s the right consistency.