Grilled Tandoori-Style Chicken Drumsticks

In a large bowl, whisk the yogurt with the ginger, garlic, lemon juice, mustard seeds, garam masala, kosher salt, and chile powder. Using a small, sharp knife, make shallow 1-inch slashes in the chicken. Add the chicken to the marinade and turn to coat. Cover and refrigerate for 4 hours. Light a grill, heat a grill pan, or preheat the oven to 450 degrees F. Remove the chicken legs from the marinade, wiping off all but a thin layer. Grill the chicken legs over moderate heat, turning occasionally until browned and cooked through, 30 minutes. Alternatively, roast the chicken legs on an oiled baking sheet for 20 minutes, turning once halfway through. Transfer the chicken legs to plates, sprinkle with the chaat masala, and serve.