Fill a wide pot with at least 1 1/2 inches of water, with a thick ring of crumpled tinfoil placed inside to act as a "booster seat." Place over high heat until steaming-hot, then adjust temperature to maintain a gentle simmer. Combine egg whites, sugar, salt, cream of tartar, and vanilla seeds (if using) in the bowl of a stand mixer. Set over steaming water, stirring and scraping constantly with a flexible spatula, until egg whites hold steady at 175°F (79°C), between 8 and 10 minutes. Transfer to a stand mixer fitted with a whisk attachment and whip at high speed until meringue is glossy and beginning to ball up inside the whisk, about 5 minutes. Use immediately.