Preheat broiler.
Soak ancho chiles in boiling water (1 cup), 5 minutes.
While chiles soak, broil tomatoes, onion slice, and garlic on a foil-lined rimmed baking sheet 3"-4" from heat, turning once, until lightly charred, 5-7 minutes.
Drain chiles, discarding soaking liquid, and add to blender with tomatoes, onion, garlic, chipotles with sauce, cumin, ½ teaspoon salt, and cold water (2 tablespoons). Purée until smooth.
Preheat oven to 350°F. Wrap 2 stacks of tortillas in foil; warm in oven while finishing filling.
Cook potatoes in a 3- to 4-quart pot of well-salted boiling water until just tender but not falling apart, 5-7 minutes. Drain.
Coarsely grind chicken in a meat grinder or food processor (or use pre-ground meat).
Heat oil in a 12" heavy skillet over medium-high heat until it shimmers. Add chile purée (it will splatter) and cook, stirring, 4 minutes (sauce will thicken).
Add ground chicken and cook, stirring and breaking up clumps, 1 minute. Cover skillet and reduce heat; simmer, stirring occasionally, until chicken is cooked through, about 4 minutes.
Gently stir in potatoes and reheat.
Serve chicken and potatoes with warm tortillas and accompaniments.
