In a large mixing bowl, beat the softened butter with a hand mixer or stand mixer on medium speed until light and creamy (about 2-3 minutes).
Add marshmallow fluff:
Add the jar of marshmallow fluff (or marshmallow creme) to the butter and beat until combined and smooth.
Add powdered sugar:
Gradually add the powdered sugar, one cup at a time, mixing well after each addition. Scrape the sides of the bowl as needed to ensure everything is well incorporated.
Add vanilla and cream:
Stir in the vanilla extract, heavy cream (or milk), and a pinch of salt. Beat the frosting on medium-high speed for an additional 2-3 minutes until it’s light, fluffy, and the desired consistency is achieved. If the frosting is too thick, you can add more cream, one tablespoon at a time, until it reaches the right consistency.
Frost your cake or cupcakes:
Once the frosting is fluffy and smooth, use a spatula or piping bag to frost your cake, cupcakes, or cookies.
