Wipe the mixing bowl and whisk attachment with lemon juice or white vinegar to ensure they are completely free of residual fat.

In the metal bowl of a stand mixer, whisk together egg whites and sugar until combined.

Place the bowl over a saucepan filled with 1 inch of simmering water to create a bain-marie, ensuring the bowl bottom does not touch the water.

Whisk the egg white mixture constantly until the sugar has fully dissolved and the internal temperature reaches 160°F (71°C) on an instant-read thermometer.

Transfer the bowl to the stand mixer and whisk on high speed until the meringue forms stiff, glossy peaks and the exterior of the bowl is completely cool to the touch.

Reduce the mixer speed to medium-low and replace the whisk with the paddle attachment.

Incorporate the softened butter one cube at a time, allowing each piece to fully emulsify before adding the next.

Add the vanilla extract and salt, then continue mixing on low speed for 2 to 3 minutes to remove air bubbles and achieve a smooth texture.