Fill a medium saucepan with 1 inch of water and bring to a simmer.

In a medium heatproof bowl, whisk together the egg whites, granulated sugar, powdered sugar, cream of tartar, and vanilla until combined.

Place the bowl over the simmering water, making sure the bottom of the bowl doesn’t touch the water. Whisk until the mixture is warm and the sugar has dissolved.

Pour the mixture into a stand mixer fitted with the whisk attachment and beat on high speed until the mixture cools to room temperature, 15 to 20 minutes.

On low speed, slowly add the butter 1 tablespoon at a time until fully combined. Scrape down the sides and bottom of the bowl as needed.