Heat butter in heavy saucepan and add crayfish, onion and garlic. Saute over medium heat until the crayfish have changed color, about 5 minutes. Add the fennel, hot pepper and water. Cover and cook at a gentle simmer for 30 minutes. Remove and discard the fennel and the hot pepper. Remove the crayfish and pound in a mortar and push through a fine sieve or reduce to a puree in an electric blender with a little of the stock. Return the pureed crayfish to the saucepan, add the coconut milk and heat through. Beat the eggs yolks with a little of the soup and stir into the saucepan. Cook over low heat, stirring constantly, until the soup is lightly thickened. The crayfish shells may be simmered the water first to make a more flavorful stock instead of using plain water.