Mulato Chiles are very similar in appearance to the Ancho chile and they are closely related. Both are Poblano chiles and the difference between the two is when they're harvested.
The color of these chiles while growing is dark green. Ancho chiles are picked when they ripen to red and they're then dried. Mulato chiles are allowed to fully ripen turning a dark brown where they're then harvested and dried. This additional ripening time adds to the Mulato's flavor characteristics and they're more full-bodied and complex in flavor than the Ancho. The added nuanced flavor gives them a special role in authentic Mexcian cuisine.
Along with the other two members of the "holy trinity" the Ancho and Pasilla Chiles, the Mulato chiles are a key ingredient in mole poblano, which is also known as Mexican mole, a dark brown chocolaty and spicy sauce that is usually served over chicken or meat. Dried mulato chiles are ideal for mole recipes due to their dark brown color after soaking.
You'll also find Mulato Chiles used in other Mexican sauces and stews, including chicken with rice. Cooks in Mexico make stuffed Mulato peppers (similar to stuffed Poblanos) by rehydrating the chile pods, removing the seeds and then stuffing the pods with breadcrumbs, cheese and shrimp. These are then pan fried in oil.
Scoville heat rating (SHU) of 2,500 - 3,000