Almond extract by John Spottiswood.

vanilla extract (use a bit more) OR almond liqueur (use 4-8 times as much) OR bitter almond 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.)