Swiss Meringue

Notes

This can be made with less sugar (175 g) and heated to a lower temperature (140 ºF or 60 ºC), but it’s arguably less stable.

170 g egg whites, from 5 to 6 large eggs
250 g plain or lightly toasted sugar
1/4 teaspoon Diamond Crystal kosher salt; use half as much if iodized
1/4 teaspoon cream of tartar
Scraped seeds from 1 split vanilla bean, or 1/2 teaspoon pure vanilla extract (optional)

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 or vanilla seeds (if using) in the bowl of a stand mixer.
Set over steaming water, whisking constantly, until sugar has dissolved and egg whites hold steady at 175 ºF (79 ºC), between 5-8 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.