Shake egg yolk only in shaker with a few ice cubes for 10 seconds. Fill shaker with ice and add egg white, gin or bourbon, buttermilk, and syrup. Shake for 30 seconds (or more) and strain into a large glass. Grate nutmeg over top and serve.
The original recipe called for gin, and that was good, but then we tried it with bourbon, and that’s even better. Your call.