Bring a few inches of water to a boil in a medium saucepan over medium-high heat. Whisk the egg whites, sugar, water, corn syrup, vanilla, cream of tartar and salt in a large heatproof glass bowl. Set the bowl on top of the pan (do not let the bottom of the bowl touch the water). Beat with a hand mixer on low speed until the sugar dissolves and the mixture starts to thicken, 1 to 2 minutes. (If any sugar crystals form on the side of the bowl, brush them down with a wet pastry brush.) Increase the speed to medium high and beat until stiff glossy peaks form, about 7 minutes.
Remove the bowl from the pan and beat until the frosting cools slightly, about 30 seconds. Use while still slightly warm -- the frosting will firm up and form a hard shell when it cools. Do not refrigerate the frosted cake or the frosting may weep.
Chocolate Seven-Minute Frosting: Make Seven-Minute Frosting. Spoon into 3 ounces cooled melted semisweet chocolate and gently fold together.