Place the sugar, water, egg whites and cream of tartar in a glass or heat proof bowl that will fit over a double boiler. Beat the mixture, off the heat, with a hand mixer until white and foamy, about 1 minute.
Place the bowl on top of the double boiler, on low heat, and beat on high for about 10 minutes until it is light, fluffy and stiff peaks form.
Remove from heat and add the vanilla extract, salt and vanilla bean seeds. Beat for another 2-3 minutes to thicken even more.
Scoop into a pastry bag fitted with a large round tip or frost using an off-set metal spatula.
Recipe Adapted from the cookbook Cupcakes! by Elinor Klivans