Blend with a cup of ice until smooth. Pour unstrained into a Pilsner glass or other tall glass.
I've found that a softer gin works well when shaking rather than blending. The orgeat can result in an unpleasantly musky nose in this drink if shaken; to avoid, omit the syrup or blend with lots of ice as in the original instructions.
Won 1967's IBA World Cocktail Championship
Jeff Berry, Beachbum Berry's Taboo Table, p. 87.