Add the egg whites, granulated sugar, cream of tartar, and salt to a heat-safe bowl of a stand mixer.
Set the bowl over a saucepan filled with a few inches of simmering water. If you aren’t using a stand mixer, use any large heat-safe bowl, with high sides for mixing.
Heat the mixture, whisking constantly until the egg whites are warm to the touch and sugar has dissolved, about 3-4 minutes.
Remove the bowl from the heat and transfer to your stand mixer fitted with the whisk attachment.
Mix on high speed, until stiff, glossy peaks form, about 7-9 minutes. Add the vanilla and peppermint during the last 10 seconds of mixing.
Spread or pipe onto cake immediately.