Place the sugar and water in a small saucepan over medium heat. Cover the pot for the first 2 minutes to help prevent crystallization. You will heat the sugar syrup to 240°F.
While the sugar heats, place the egg whites in the bowl of a stand mixer fitted with a whisk attachment. When the sugar syrup hits 200°F, begin whipping the egg whites on low. Once they start to foam, add the salt and cream of tartar.
Bump the speed up to medium (4 on a KitchenAid) and beat until you reach soft peaks. The whites should be ready right when your sugar syrup is ready. If you hit soft peaks too soon, turn the mixer to the lowest speed and allow it to mix until the sugar syrup is ready.
Once the sugar is 240°F, turn the mixer to a medium speed. Slowly and carefully drizzle the syrup into the bowl, being careful to avoid the whisk. Continue to beat the meringue on a medium speed until it is thick, glossy, and forms stiff peaks (about 10 minutes).
Stop the machine and remove the whisk. Use a kitchen torch to toast the meringue so that it is a deep golden brown color. Replace the whisk attachment and whisk to combine. Repeat this process until the meringue is toasted to your liking, about 5- 8 times or so.
Begin adding the butter one tablespoon at a time. Wait about 10 seconds between additions. The mixture will deflate and look loose, but it will thicken up again.
Once the butter has been added, scrape down the bowl and whip on a medium high speed for 3 minutes. Scrape the bowl down, add vanilla, and whip to combine.