1/2c.marshmallow creme (also known as marshmallow fluff)
1-4tbsp.milk
Instructions
In bowl of stand mixer, whip butter with whisk attachment until light and fluffy, 3-4 minutes.
Sift powdered sugar. Add to butter, 1/2 cup at a time. Mix in slowly then increase speed to high to whip.
Measure 1/2 cup marshmallow creme and stir on medium until combined and thick.
If frosting is too thick, add milk 1 tablespoon at a time. If it's too thin, add powdered sugar, 1/4 cup at a time until frosting reaches desired consistency.
Notes
Note: Frosting may be just a bit softer than a typical stiff buttercream, due to marshmallow creme. I recommend piping frosting shortly before serving, in order to preserve the look and stiffness of the frosting.