Working quickly, in a clean metal bowl, add the egg whites, salt, and cream of tartar. Beat on medium speed until they reach the soft peak stage, then add in the sugar, one tablespoon at a time, incorporating each one fully before adding the next. Continuing at medium speed, beat the whites until they become stiff and glossy, and when you turn off the mixer, the mixture holds a stiff peak. Do not overbeat, or they will become weepy. Gently spoon the meringue evenly between the two baking pans, over the cake batter.