Whisk the egg whites and sugar in a metal bowl for a minute .

Place bowl over a saucepan with an inch of water (double boiler method) and bring the water to a simmer while whisking constantly.

Whisk and heat until all the sugar has dissolved , or a thermometer shows 160 F / 70 C.

Take off heat (And saucepan) and start beating at medium high speed until stiff peaks are formed and the outside of the bowl is cool.

Add butter , one cube (tablespoon) at a time while whipping. Repeat with all the butter.

Beat at high until desired consistency is achieved.

Add the vanilla and beat for 3-5 minutes more to remove any air bubbles.