In the bowl of a stand mixer, whisk together egg whites and sugars by hand. Place bowl over a saucepan of simmering water. Cook, whisking occasionally, until an instant-read thermometer registers 155°F (68°C) to 160°F (71°C).

Carefully return bowl to stand mixer fitted with the whisk attachment, and beat at high speed until bowl is cool to the touch, about 8 minutes. Add butter, 2 tablespoons (28 grams) at a time, beating until combined after each addition. Beat in vanilla and salt. Use immediately, or refrigerate in an airtight container for up to 3 days. If refrigerating, let stand at room temperature for 2 hours and rewhip until smooth before using.