In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with a hand mixer, place the butter, milk, and vanilla, and beat on medium speed until combined.
Raise the speed to high and beat until creamy. Add the salt, meringue powder, and about 3 1/2 cups of the confectioners’ sugar. Mix slowly until the sugar is incorporated. Beat at high speed until uniformly thick. Add up to the remaining 1/2 cup of the confectioners’ sugar, if necessary, to thicken the frosting.