Boil the crab legs or crabs for about 10 minutes, then drain and cool; remove the claws from the crabs, if using, and reserve the bodies for another purpose.
Have the shrimp paste ready; preheat the oven to 350 degrees.
Pour the oil into a bowl.
Dip your fingers into the oil and pick up 2 tablespoons of the shrimp paste.
Mold it into an oval around and halfway down the crab claw, covering the part of the claw where it was attached to the body; this will leave a claw tip extended to serve as a handle.
Place the claws on a baking sheet and bake in the preheated oven for 30 minutes.
Serve with Nuoc Cham and watercress.
Note: In Vietnam, this dish is always barbecued over charcoal.
If you wish to prepare it this way, cook for 10 minutes on each side.