Add the egg whites, sugar, corn syrup, water, salt, and cream of tartar to the top of a double boiler.
Place top pot over rapidly boiling water.
Beat the egg mixture with a hand mixer on low for 30 seconds.
Increase the speed to medium-high, and continue mixing until the egg mixture stands in peaks (see the picture above the recipe).
Remove from heat.
Mix in vanilla extract.
Use immediately to frost your cake or cupcakes.
Refrigerate any leftover frosting.
Egg whites: I use cold, from the fridge egg whites. It's really important that you don't get any yolk in the egg white when separating, though, because the frosting won't whip up right with yolk in it.
Corn syrup: Unfortunately, I don't have a good substitute for the corn syrup in the recipe.
Double boiler: If you don't have a double boiler, check out my section about the recipe for tips on faking a double boiler.
Salt: A couple pinches of salt help to add balance to the frosting.
Cream of tartar: The cream of tartar helps to stabilize the frosting.