Pound in a mortar the shells and trimmings of crayfish cooked a la mirepoix (the tails being used for some other dish). Add the same amount of fresh butter and blend well. Put the mixture in a small saucepan standing in a larger pan of hot water, allowing the crayfish butter to melt slowly.
When it is completely melted, pour it through a cloth secured over a bowl of iced water. Twist the cloth to squeeze out all the butter. This will sloidify in the icy water, when it can be spooned out of the bowl and dried on a muslin cloth.
Use it to add to sauces, gravies, thick shellfish soups, fish forcemeats and shellfish ragouts. Crayfish butters form the basis for the sauce called a la Nantua.