Remove most of the flesh of each avocado half with a spoon, leaving 1 cm attached to the shell. Place the flesh in a large bowl. Mash the avocado with a fork and add the lime juice. Season with salt and pepper and set aside.
In a large frying pan, heat the butter and sauté the prawns for 2 minutes. Set a few prawns aside for a garnish and add the rest to the mashed avocado. Fill the avocado skins with the prawn-avocado mixture, sprinkle with grated cheese, and heat under the grill until the cheese turns golden. Garnish with the reserved prawns and serve.