Prepare your mixing bowl and whisk attachment; ensure they are clean.

In a heatproof bowl, combine egg whites and sugar over simmering water, whisking until the sugar dissolves and the mixture feels warm.

Transfer to a stand mixer and beat on medium-high speed until stiff peaks form (about 8-10 minutes).

Gradually add room temperature butter one tablespoon at a time while mixing on low speed until fully incorporated.

Mix in vanilla extract or any desired flavorings to taste.