2 oz. Pedro Ximinez Sherry
1/2 oz. Angostura bitters
1 whole egg
nutmeg, for garnish
Shake hard with ice, strain into a chilled cocktail glass, and garnish with freshly grated nutmeg. For the sherry I suggest the Lustau San Emilio PX, which is balanced by more acidity than some other PX sherries.
[Photo courtesy of Culinate, from my article about sherry cocktails.]