In the bowl of a stand mixer fitted with the paddle attachment, beat butter at medium speed until creamy. Beat in vanilla and salt. Reduce mixer speed to low, and gradually add confectioners’ sugar. Increase mixer speed to medium, and beat until smooth. Beat in milk, 1 tablespoon (15 grams) at a time, until frosting reaches a spreadable consistency.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/spumoni-cake/