Place the egg whites and sugar into a metal bowl (I use my standmixer's metal bowl) and set it over a bowl of boiling water, not touching the water.

Stir gently until all the sugar has melted. (It should turn almost white)

Whip the mixture until stiff, normally about 5 minutes.

Transfer the bowl into ice-cold water and cool the mixture before adding the butter.

Turn the machine on slow and whip in the butter one at a time, ensuring the butter is mixed in before adding the next.

When all the butter have been added, turn on medium and whip until the mixture becomes no longer watery and has the consistency of cream.

Use as desired immediately, or store in the refrigerator for up to 3 days.