In a double boiler, over high heat bring egg whites and sugar to160°

(A double boiler is when you have a pan of boiling water with a bowl on top without the water touching it to gently cook the item inside, in this case, egg whites and sugar)

Stir or whisk until the mixture reaches 160°.

In the bowl of a mixer whip the egg white mixture to stiff peaks and keep mixing until the mixture cools, this takes about 10 minutes. The sides of the bowl shouldn't be warm if they keep mixing.

Then on medium speed add in the butter 1 tablespoon at a time, the mixture may look clumpy, separated, or look runny, just keep mixing.

It will then become thick and when spread with a spatula it's smooth.

If it's runny, put the bowl in the fridge for 10-15 minutes.

Then mix more until it's thick and smooth.

Add in vanilla extract and mix to combine.