Boil the crabs for thirty minutes, and drain; when cold break them apart, pick out the meat carefully, scrape off all fat adhering to the upper shell, and save these for deviled crabs.
Set the crab meat aside; put the under shell and the claws in a mortar with the butter and rice, and pound them as smooth as possible; then put this into a saucepan, and add salt, herbs, peppers, mace, and stock; boil slowly for one hour, pour it through a sieve, and work as much of the pulp through the sieve as possible. Place the soup on the range to keep warm, but not to boil.
Beat up the yolk of the egg, and add it slowly to the warm milk previously boiled; whisk the milk into the soup; taste for seasoning. Now take the crab meat and heat it in a little boiling water, drain, put it into a hot soup tureen, pour the soup over it and serve.