Such a delight to discover a widely used Mexican herb, hoja santa or Mexican pepperleaf, that has nearly the same taste as sassafras! It helps me bridge the cultural gap between Pennsylvania, where I brewed this, and Oaxaca, whose mole negro recipe inspired this twist on chocolate stout.
Cinnamomum spp. (Some of the old recipes specify cassia.) For brewing, you generally want the whole stick (inner bark), and depending on how much of the taste you want in the final product, can either add it to the last 15 minutes of the boil, the primary, and/or the secondary.
The rather extensive Wikipedia entry includes this bit: “Cinnamon is used as a flavouring in some alcoholic beverages, such as Fireball Cinnamon Whisky. Cinnamon brandy concoctions, called ‘cinnamon liqueur’ and made with distilled spirits, are popular in parts of Greece.”