Separate eggs using the three-bowl method: crack two eggs into the middle bowl and separate yolks from whites, weighing the whites.

In the stand mixer bowl, combine egg whites and sugar.

Place the bowl over simmering water, ensuring the water doesn't touch the bottom of the bowl.

Add a metal spatula between the bowl and pot to prevent suction.

Optionally, add a pinch of cream of tartar to help the meringue whip up.

Whisk continuously until the egg white mixture reaches 165°F/71°C.

Test the mixture by touching it to ensure the sugar is melted and it's warm but not too hot.

Transfer the bowl to the stand mixer and attach the whisk attachment.

Whip until the mixture becomes ribbony and the bowl is just warm.

Gradually add room temperature butter, a few tablespoons at a time, fully incorporating before adding more.

Scrape down the sides of the bowl to ensure all ingredients are well combined.

Switch to the paddle attachment and paddle on medium speed for 5 to 10 minutes to eliminate larger air bubbles and achieve a smoother frosting.