Whisk together the egg whites, sugar, salt, and cream of tartar in the top pan of a double boiler (or, make a homemade version by placing a heatproof bowl over a sauce pan filled with 2 to 3 inches of simmering water, ensuring that the bottom of the bowl does not touch the water). Cook over medium-high heat, whisking intermittently, until the mixture registers 160°F on a candy thermometer.