Instructions:
- Prepare the Bain-Marie: Fill a saucepan with 1-2 inches of water and bring it to a gentle simmer. Ensure the bottom of your heatproof bowl does not touch the water.
- Combine & Heat: Place the egg whites and sugar into the heatproof bowl over the simmering water. Whisk constantly until the mixture reaches 160°F (71°C) and no sugar granules can be felt when rubbed between two fingers.
- Whip to Stiff Peaks: Immediately transfer the hot mixture to a stand mixer fitted with the whisk attachment. Start on medium speed, gradually increasing to high, and whisk until the meringue is glossy, thick, forms stiff peaks, and the bowl feels cool to the touch.
- Cool the Meringue: Continue mixing on medium speed while the meringue cools down significantly. Do not proceed until the meringue is completely cool.
- Incorporate the Butter: Switch the mixer speed to low. Begin adding the room-temperature butter, one cube at a time, ensuring each piece is mostly incorporated before adding the next.
- The Break Phase: The mixture may initially look curdled or soupy; this is normal. Continue beating.
- Whip to Emulsify: Increase the mixer speed to medium-high and continue beating for another 3–5 minutes. The mixture will suddenly transform into a pale, incredibly smooth, and voluminous buttercream.
- Finish: Stop the mixer, scrape down the sides, add the vanilla extract and salt, and mix briefly on low speed to combine fully.