Preheat oven to 325°F. Set up a double boiler. Combine egg whites and powdered sugar in a mixing bowl and warm over the double boiler until the powdered sugar has dissolved and the mixture is warm to the touch. Place the sugar-egg white mixture on a stand mixer and mix on medium speed with the whisk attachment. Add the vanilla and salt. Whisk to medium to stiff peaks. Fold in the chocolate. Pour mixture into a piping bag, and dispense meringues onto a parchment-lined baking sheet, leaving about 1 inch between meringues. Bake 15 to 20 minutes. If meringues start to brown, turn the oven down. Meringues are ready when they can be picked up off the tray easily without sticking. Cool completely and store in a tightly sealed container.