In a heatproof bowl set over (not in) a saucepan of simmering water, combine egg whites, sugar, salt and water. Cook over medium, stirring constantly, until sugar has dissolved (or mixture registers 150 degrees on an instant-read thermometer), 2-3 minutes.

Transfer to a large bowl. Using an electric mixer, beat on medium-high until glossy, stiff peaks form (do not overbeat!), about 3 minutes; reduce speed to low and add vanilla extract and beat until just combined.

Use immediately.

Chocolate Variation: top frosted cake with shaved dark chocolate.

Lemon Variation: substitute 2 tablespoons fresh lemon juice for vanilla extract.