The Buffet Blue Book by John H. Considine (1912, p.50)

0.5 large bar glassice base
1 jiggerMissouri applejack or brandy base
milk — Fill up.dilution
0.5 teaspoonfulsweet chocolate or cocoa other
0.5 tablespoonfulsugar sweetener
  1. Shake the ice, sugar, sweet chocolate or cocoa, Missouri applejack or brandy, and milk in a large bar glass.
  2. Strain into a large stem glass and serve.

Ice: Ice

See also: Milk Punch, Chocolate Punch

Source recommends preparing a bottle of cocoa or chocolate sweetened in liquid form for convenience.