1) In a medium-sized mixing bowl, whisk together the meringue powder, salt, and confectioners' sugar. Add 1/3 cup cool water and the vanilla and stir, or beat on slow speed. The mixture will seem hard and lumpy, but the sugar will dissolve after 4 or 5 minutes and everything will smooth out.
2) Add more water, 1 tablespoon at a time, mixing well after each addition to achieve a spreadable consistency. For a very smooth, shiny glaze, the icing should be the consistency of corn syrup or molasses.
3) To use, dip the tops of cooled cookies in the glaze, then sweep a spatula over them to remove the excess. Place the cookies on a rack for several hours for the glaze to harden and dry. This may take as long as overnight, depending on the humidity of your kitchen and the consistency of the glaze.
Yield: 1 1/4 cups glaze.