Tonka beans
Tonka beans are the seeds of the South American tree Dipteryx odorata. The aroma is complex, with notes of vanilla, almond, cinnamon and fresh-cut hay; its source is coumarin.
They are used in very small quantities: grated finely straight into a mixture, or infused whole in hot cream or milk. One bean is enough for several kilograms of product — too much turns bitter and cloyingly perfumed.
The legal position needs checking. Coumarin is hepatotoxic in high doses, so tonka beans are legally restricted. In the United States the FDA prohibits their use in food. In the European Union, maximum coumarin levels in finished products are set by flavouring regulation. If you sell what you bake rather than baking for yourself, verify the rules of your market before putting tonka beans into a recipe.
Store them airtight: the aroma is volatile and fades quickly.