1) To make the base: Dissolve the sugar and salt in the boiling water, and cool to room temperature.
2) Mix in the meringue powder. Use a mixer to beat on low speed for several minutes, until the powder is dissolved and the mixture is foamy. Increase the speed and beat until soft peaks form.
3) Beat in the confectioners' sugar. Add the soft butter a few tablespoons at a time, beating well after each addition.
4) Divide the base into three roughly equal quantities in three separate bowls; if you have a scale, each should weigh about 13 1/2 ounces.
5) Add the separate flavorings to each of the bowls, beating until evenly incorporated.
6) Pipe or spread the three layers of frosting onto 12 cupcakes.
Yield: enough frosting for at least 12 cupcakes.