How to Make It
In a skillet, toast the coriander, cumin, fenugreek, mustard seeds and chiles de árbol over low heat, stirring, until very fragrant, about 5 minutes. Let cool slightly, then grind in a spice grinder.
In a blender, mince the garlic, ginger, turmeric and serranos with 1/4 cup of the oil. Add the cilantro and the remaining 1/4 cup of oil and puree to a coarse paste. Stir in the spice mixture and 1 teaspoon of salt.
In a bowl, massage the marinade all over and under the chicken skin. Refrigerate at least 4 hours or overnight.
Light a grill or preheat a grill pan and brush with oil. Grill the chicken over medium-low heat, turning, until golden and cooked through, about 40 minutes.
In a saucepan, cover the potatoes with water and add the butter and saffron. Simmer over moderate heat until tender, about 15 minutes. Drain and season with salt.
Arrange the chicken on the potatoes on a platter and garnish with mint and cilantro. Serve with the seasoned yogurt.