There are two versions of Crayfish Butter in the classical French canon.
Cold version (à froid):
1 part cooked crayfish puréed, blended with an equal amount of butter, then pressed through a sieve.
Use for cold hors d’oeuvres, cold fish and fish sauces.
Hot version (à chaud):
1 part cooked crayfish puréed, blended with an equal amount of butter, then melted in a bain marie, then strained through a cloth into a bowl filled with iced water. The melted, strained butter will congeal: collect it out of the water and put in on a cloth to dry.
Use in sauces, soups and seafood stews, and to make Nantua Sauce.
For both versions of Crayfish Butter, use the shells and trimmings of crayfish, finely chopped up and cooked in a mirepoix.