Source: The Food Lab, page 259

6 ancho, pasilla, or mulato chiles
3 New Mexico red, California, costeño, or chicorico chiles
2 cascabel, árbol, or pequin chiles

Seed the chiles and tear them into chunks

Toast over medium-high heat until slightly darkened, 2-5 minutes

2 cup stock

Add to chiles, simmer until softened, 5-8 minutes

Blend, adding water if too thick