Beat butter till smooth and creamy. Add about half of the raspberries, the salt, and the vanilla.
Beat in about half of the powdered sugar. Continue beating in raspberries and powdered sugar till you get a smooth consistency.
The raspberries will thin the frosting, the powdered sugar will thicken it.
Notes
-I used fresh raspberries, but I think frozen raspberries would work as well. They tend to be more juicy, so start with a small amount first. And of course thaw them before you add them so you don't break your mixer! :)