Like vanilla, tonka beans (dipteryx odorata) are all about fragrance, or fragrance expressed as flavour; if you have a poor sense of smell, you might wonder what all the fuss is about. The first impression is vanilla, followed by bitter almond, cherry pits, maybe a hint of cloves, with a pervasive overall bitter-sweetness, like caramel, or burnt sugar.
It is lovely in ice cream, crème brûlée, panna cotta, and such; compliments pretty much anything that vanilla does – chocolate, fruits, nuts – and vanilla itself. And like vanilla, it is also lovely, used with restraint, in savoury dishes such as braised meat and poultry (a spin on poule à vanille perhaps), delicate seafood dishes (scallops, lobster), in mayo, mashed potatoes…
Also brilliant in drinks and cocktails; it is used in Berliner weisse, and it would be very interesting in dark spirit based cocktails.
To use, grate as you would nutmeg, either to infuse in a liquid, or apply directly atop to finish.
Oddly, tonka beans are not legal for consumption in the U.S. because they contain coumarin, which gives them their fragrance, and which, in impossibly high quantities, is toxic.