Servings: cups (Enough for 1 double-layer 8" round cake)
Whisk together egg whites and sugar in a double boiler. Or, in a metal bowl of a stand mixer over a slightly boiling pot of water. With medium heat from the steam, continue whisking until the sugar is completely dissolved - test by drawing up whisk and feeling for sugar crystals in the egg white. When completely smooth, transfer bowl to stand mixer.
Using whisk attachment, whisk on slow until whites begin to form then increase speed to medium high. Continue whisking until bottom of bowl and top of bowl feel the same temperature - this could take 15 minutes. Be patient.
Add cool, yet soft butter one bit (1/2 Tbl) at a time..one by one while whisking on medium high. Again, patience.
Add tinting gel, if desired.
Hot tip: Margarine is not a very good substitute. Vegetable shortening is a doable substitute for half the butter - the flavor is different and careful if adding coloring.