Preheat the oven to 225°. In a small bowl, mix sugars. In the bowl of an electric mixer, add egg whites. Beat until foamy, then add cream of tartar and salt. Add vanilla to incorporate.
Start adding sugar to the mixture slowly, 1 tablespoon at a time. The egg whites should appear stiff and glossy.
Transfer mixture to a piping bag fitted with a large star tip. Line 2 baking sheets with parchment paper. Pipe meringues onto baking sheet.
Bake for 1 hour exactly, then turn off oven and let meringues sit in the unopened oven for three hours, and up to eight.