In a large mixing bowl, dissolve the granulated sugar in the boiling water; let cool completely.
Add the meringue powder, and beat using the whisk attachment until soft peaks form. Add the confectioners' sugar, mixing until thoroughly combined.
Add the butter a few tablespoons at a time, beating constantly. The frosting will become very creamy. Beat in the vanilla, lemon powder (or zest), optional Fiori di Sicilia, and salt.
Use buttercream immediately to fill and frost a cake. Or store it, covered in the refrigerator, for up to 2 weeks. Freeze for longer storage.
Tips from our Bakers
Don’t want to use meringue powder? For a classic buttercream made with fresh egg whites, try our Swiss Buttercream recipe. Or amend this Lemon Buttercream recipe as follows: Substitute 4 large whites, at room temperature, for the meringue powder. Combine the whites with the cooled sugar-water mixture and beat until foamy, then add 1/2 teaspoon cream of tartar. Continue to 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.