1. In a large mixing bowl, dissolve the granulated sugar in the boiling water; let cool completely.

  2. Add the meringue powder, and beat using the whisk attachment until soft peaks form. Add the confectioners' sugar, mixing until thoroughly combined.

  3. 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.

  4. 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.