In a small bowl, stir together the lime zest, chile powder, paprika and salt. Sprinkle half of the spice mix over the flesh side of the salmon. Reserve the remaining spice mix for serving.
Heat the oil in a 12-inch ovenproof nonstick skillet over high heat until hot. Add the salmon, skin-side up and cook until the salmon begins to brown around the edges, about 2 minutes. Flip the salmon. Transfer the skillet to the oven and roast until the thickest part of the salmon is firm to the touch, about 8 minutes.
While the salmon cooks, make the sauce: Stir together the yogurt, mayonnaise, lime juice, cilantro, scallions and serrano in a medium bowl.
Place the salmon on individual plates or a serving platter. Dollop each fillet with the sauce and sprinkle with the reserved spice mix.