As much as you may eschew using shortening, it?s critical here, as it prevents the frosting from becoming unpleasantly soft in warm weather.
1/2 cup (4 ounces) unsalted butter
1/2 cup (3 1/2 ounces) non-trans-fat vegetable shortening
a scant 1/4 teaspoon salt
2 tablespoons meringue powder (optional, for a smoother icing that holds its shape a bit better)
2 teaspoons vanilla extract
6 cups (24 ounces) confectioners' sugar or glazing sugar
4 to 6 tablespoons (2 to 3 ounces) milk