"Peppermint" is a plant that has a strong and refreshing smell. It is often used to make candies, gum, or flavor things like tea or toothpaste. Full definition
If you only have extract, the general conversion rule is 1 part oil to 4 parts extract, which would mean for about 1/2 teaspoon of peppermint extract for this recipe. (organicallymandy.com)
If you feel like mixing it up, try substituting the 1 teaspoon of vanilla extract with peppermint extract. (laurasglutenfreekitchen.com)