Peppermint extract by John Spottiswood.

peppermint oil (One part flavoring oil is roughly equivalent to four parts extract, but this may vary according to the products used. To be safe, begin by substituting 1/8 teaspoon of oil per teaspoon of extract, then add more drops of oil until you're satisfied with the flavor.) OR creme de menthe (use much more) OR peppermint schnapps (Substitute 1 or 2 tablespoons schnapps for each teaspoon of extract) OR vanilla extract (use more)