In a small bowl combine the vegetable oil, chili powder, garlic powder, salt and cayenne pepper to prepare rub.
Using 1 cod fillet (8-12 oz, about 1/2-inch thick, cut in half) or 2 cod tenderloins (6 oz each, about 1-inch thick), spoon rub on top of cod, spreading evenly. If needed, tuck any thin parts of fillet underneath for even cooking.
Put prepared cod in Ziploc® Brand Zip 'n Steam® Medium or Large bag, placing thicker portions toward outer edge of bag. Seal bag and place in microwave.
Cook on full power for 3 1/2 - 4 minutes or until cod is cooked through and flakes easily. Use 3 1/2 minutes for 1/2-inch thick pieces and 4 minutes for 1-inch thick pieces. If needed, continue microwaving at 30-second intervals until cod is done.
Allow bag to stand 1 minute before handling. Carefully open bag. Use spatula to remove cod from bag.
Spoon 2 Tablespoons freshly squeezed lime juice over cod. Garnish with 1 Tablespoon chopped fresh parsley and serve with 2 Tablespoons melted butter, if desired.