Melt the butter in a saucepan over medium heat. Add the marshmallows and stir continuously with a spatula until dissolved.
Sift the confectioners' sugar and salt into a bowl. Add the marshmallow mixture and whisk to combine. Whisk in the vanilla. Thin with milk as needed to achieve the desired consistency. Use warm.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.