Season the crabs with salt and pepper, and sprinkle both sides with the chili powder. Preheat a pan with the oil until nearly smoking. Add the crabs, belly-side down. Cook for 5 minutes before turning. Lower the heat and cook for another 5 minutes on the second side. Remove from the pan and pat dry with paper towels to remove any excess oil.
Build your sandwich with lettuce, tomato, sprouts, and avocado, topped off with the crab and chili mayonnaise.