1) Dissolve the sugar (and salt, if you're using it) in the boiling water, and cool to room temperature.
2) Mix in the meringue powder. Use a mixer to beat on slow speed for several minutes, until the powder is dissolved and the mixture is foamy.
3) Increase the mixer speed, and beat until soft peaks form.
4) Beat in the vanilla, then the confectioners' or glazing sugar.
5) Add the soft butter or shortening a few tablespoons at a time, beating well after each addition and scraping down the sides of the bowl occasionally.
6) Add the white chocolate the same way as you did the butter or shortening.
Yield: Enough frosting to fill and frost an 8" layer cake or 24 cupcakes.