Begin by placing the softened butter and marshmallow fluff in a large mixing bowl or the bowl of a stand mixing fitted with a paddle attachment. Beat together on low for about 2 minutes until light, creamed and well incorporated.
Sift half of your powdered sugar into the bowl and beat until just combined.
Finally, sift the remaining powdered sugar into the bowl and add the vanilla. Continue beating on a low setting until creamy and smooth (see note).
As with other American style buttercreams, you can adjust the consistency of this marshmallow frosting. To make the icing thicker, add a bit more sifted powdered sugar. If your buttercream is too thick, you can add a little milk. Add any additional liquid slowly and a small amount (no more than a tablespoon) at a time. Scrape down the sides of the bowl occasionally with a spatula to make sure that you have mixed everything together and not missed any pockets of powdered sugar.