Beat together marshmallow fluff and butter in large mixing bowl with electric mixer on medium speed until well combined.
Beat in vanilla and salt. Beat in confectioners’ sugar until well combined.
Beat in milk a little at a time if needed, until frosting just comes together but only if needed.
SNAPPY TIPS: Make sure your butter is completely softened so you don't get lumps in your frosting. This frosting makes about 4 cups which should be enough for a thin layer on a cake. If you have a lot of cupcakes or a large cake, you may want to double the recipe. If you add any mix-ins to the frosting, you'll get more frosting as well.SNAPPY SUBSTITUTIONS: Do not substitute marshmallows or melted marshmallows for the marshmallow fluff.