When making clarified butter you get a bottom layer of milk solids, the center layer of clarified butter, and the top layer of foam. What is in the foam? Does it have any uses?
That foam is more of the milk solids, brought up and made foamy by water evaporating. The Joy of Baking says it is whey protein specifically.
I usually discard it but you can eat it. Some people like it on toast or other places where you might use butter or a light sprinkling of mild cheese.