Make the frosting. In a large bowl with a hand mixer, or a stand mixer with the whisk attachment, beat the butter and melted marshmallows together on medium-high speed until light and fluffy, about 4 minutes. Add the powdered sugar 1 cup at a time, keeping the mixer on low speed until all the sugar is added. Add the vanilla and salt. Beat on medium-high speed for 3 minutes to fully combine. If the frosting looks too thick or thin, add 1 tablespoon of milk or powdered sugar, depending on need....
