dried árbol chiles
A small, slender, bright red dried Mexican chile, clearly hot, with a nutty, slightly grassy note.
The chile de árbol is a narrow chile about the length of a finger that keeps its vivid red colour when dried. Its flesh is thin and barely sweet; what stands out is a clean, fairly high heat and a nutty, slightly smoky flavour that becomes more pronounced when the chile is toasted in a dry pan. Because it adds little body, it is used mainly for heat in salsas, oils and braises rather than as the base of thick sauces. It scorches very quickly and turns bitter, so it is toasted for only a few seconds. It can be found in Latin American shops and through mail order, and sometimes in well-stocked spice shops.
Substitute:
Dried cayenne chiles or the thin dried red chiles sold in Asian shops match the heat and colour fairly well but lack the nutty note. Chilli flakes will do in a pinch, though they give less flavour and cannot be toasted or dosed in the same way.