Since 1605, Chartreuse has been produced in France by Carthusian monks from a secret recipe that includes 130 plants. Green Chartreuse, the original product, rings in at 110 proof and is stronger and less sweet than the lesser-known yellow variety. It adds spicy, herbal, and sweet characteristics to this cocktail. Green Chartreuse is also gluten-free and vegan. (In case you're wondering, Yellow Chartreuse is gluten-free but not vegan since it includes honey.)