Step 1

In a large glass or metal bowl, combine the egg whites and sugar.

Step 2

Set the bowl over a pot of barely simmering water, and stir continuously, until the sugar is completely dissolved. (Check by rubbing a small amount between your thumb and forefinger; it should be hot to the touch, and feel smooth, not gritty.)

Step 3

Remove the bowl from the simmering water and whip on high speed until the meringue is thick, white, glossy, and can hold stiff peaks. (It should be cooled completely. There should be no hint of warmth when you place your hand on the side of the bowl. Refrigerate the meringue for 15 to 20 minutes if necessary.)

Step 4

Add the butter in, a tablespoon at a time, beating well after each addition. (If the mixture begins to look curdled, continue to beat until it comes back together, before adding the remaining butter.)

Step 5

Fold in the salt and any other flavorings you may be using.