Dissolve the sugar in the water. Cook without stirring to 255° (the hard-ball stage).
While the syrup is cooking, add the salt to the egg whites and then beat them until frothy.
Remove the syrup from the flame when it forms a hard ball and pour gradually over the egg whites, beating constantly. Add the vanilla. Continue beating until candy cools and will hold its shape. Drop by spoonfuls onto a buttered pan and mark into squares.