Cut each cooked crayfish in half lengthwise. You can ask your fishmonger to do it for you if you prefer. Carefully remove the flesh from the tail. Detach and discard the intestine. Dice the flesh into 1½cm squares and refrigerate until required.
Place the four half crayfish shells in a gratin dish.
Heat 50g of the butter in a small saucepan. Add the shallots and stir until almost golden. Stir in the flour and cook over low heat for a few minutes.
Whisk in the white wine. When it is well incorporated, gradually whisk in the milk and cook over low heat for 5 minutes. Season with salt, pepper and cayenne pepper. Gently mix in the crayfish pieces and the parmesan and reheat for 2 minutes.
Preheat the oven to 220°C.
Spoon the crayfish and sauce into the four empty crayfish shells. Sprinkle the top with breadcrumbs and dot a little butter evenly here and there. Place in the hot oven and brown the top of the crayfish. Serve immediately.