Marshmallow fluff frosting is perfect for cakes, cookies, and more. It reminds me of the filling of our favorite snack cakes. Make a double batch to frost a layered cake, or use written for a 9x13-inch cake. We love it on oatmeal cream pie bars, or it would be amazing as a filling for whoopie pies.
In your mixer bowl, cream the butter until it is light and fluffy. (Go ahead and let it go for a minimum of 2 minutes.)
Add in the powdered sugar, vanilla extract, and marshmallow fluff and mix on low until incorporated. Turn the mixer to medium speed and beat for about 4 minutes.
Spread on your cooled dessert or store in an airtight container until ready to use.