Step 1

Place chiles in a medium bowl and pour in hot water to cover. Let sit until chiles are very soft, about 30 minutes; drain. Blend chiles, ginger, garlic, yogurt, cilantro stems, lime juice, mustard oil (if using), coriander, paprika, mango powder (if using), fenugreek leaves, garam masala, cumin, and 2 Tbsp. vegetable oil in a blender until smooth. Transfer marinade to a large resealable plastic bag. Season chicken generously with salt and add to marinade. Seal bag and turn to coat chicken. Chill at least 2 hours.

Step 2

Let chicken sit at room temperature 1 hour before grilling.

Step 3

Prepare a grill for medium heat; oil grate. Remove chicken from marinade, letting excess drip back into bag, and grill, turning every minute or so, until beginning to char in spots, 8–10 minutes total. Transfer to a platter and let rest 5–10 minutes before serving.

Step 4

Do Ahead: Chicken can be marinated 12 hours ahead. Keep chilled.