Whisk together flour, salt, pepper and cayenne, if using.
Pat your crabs dry and dredge in flour mixture, shaking off excess (make sure crabs are completely coated).
Heat oil and butter in a saute pan over medium-high heat. The oil should be very hot but not smoking.
Place the flour-coated crabs, back side down, in the pan and let cook for 3 minutes (pan will likely spatter, so step away or cover with a splatter screen if you have one). When you turn the crabs over, they should look golden and crisp. Cook on the second side for another 3 minutes.
Squeeze the juice of half a lemon over the crabs, sprinkle with parsley and serve.
Notes
If cooking a large number of crabs in batches, keep cooked crabs warm in a low oven.