Toast pepitas (see Tip). Place in a small bowl with butter, lime zest, lime juice and chili powder.
Generously coat a large nonstick skillet with cooking spray and place over medium heat. Sprinkle salmon with salt and pepper, add to the pan and cook until browned and just cooked through in the center, 2 to 4 minutes per side. Remove the pan from the heat. Transfer the salmon to a plate. Add the butter-lime mixture to the hot pan; stir until the butter is melted. Serve the salmon topped with the sauce.