Whiskey, lemon juice and egg white shaken until the foam sits on top, with a few drops of angostura over it. The egg white is what makes it a sour rather than a whiskey with lemon in it. Two glasses, five minutes.
Two things decide how this comes out. The first is shaking hard enough: the egg white needs real work to foam, and a timid shake gives you a thin drink with a sad film on top. The second is making them one at a time. Doubling the quantities in the same shaker gives the liquid too much room and the foam never sets properly, which is why the original note insists on it. If raw egg is a concern, pasteurised egg white in a carton works identically.