Dissolve the sugar (and salt, if you're using it) in the boiling water, and cool to room temperature.
Use a mixer on low speed to beat in the meringue powder, until the powder is dissolved and the mixture is foamy.
Increase the speed and beat until soft peaks form.
Beat in the vanilla, then the confectioners' or glazing sugar.
Add the soft butter or shortening a few tablespoons at a time, beating well after each addition. Frost cake immediately; or store buttercream at room temperature, covered, for a few hours before using.
Tips from our Bakers
If you want to use fresh egg whites instead of meringue powder, you'll need 4 large whites, at room temperature, plus 1/2 teaspoon cream of tartar. Add the whites to the cooled water and sugar mixture; beat the mixture until foamy, then add the cream of tartar and beat until soft peaks form, as directed. If you're using pasteurized egg whites from a carton, use 1/2 cup liquid whites. The cream of tartar is critical and may need to be increased to 3/4 teaspoon in order to create a stable meringue.
- For coconut frosting, add 4 to 5 drops coconut flavor and 1/4 cup (1 ounce) coconut milk powder along with the confectioners' sugar. Or flavor to taste with almond extract; coffee flavor; 2 teaspoons espresso powder dissolved in the boiling water; butter-rum flavor, or 2 tablespoons fruit purée, added after the confectioners' sugar. Be aware that adding fruit or espresso will change the color of the frosting.