I misread the recipe and used cognac in place of bourbon when I first made this cocktail. Realising my mistake, I finished making the cocktail and then...
Such a wonderful cocktail, the recipe of which irresistibly invites experimentation. So we made a couple of variants to include il Moscato di Nonino Grappa. In one, we simply substituted Grappa for the Italian Brandy and reduced the volume from 20ml to 15ml. In the other, we did the same but also substituted Cognac (Martel VS) for the Bourbon (much as in our good host's happy mis-step). Hard to say which one we enjoyed the most. Perhaps the latter.