Bring a large pan of well-salted water to the boil, drop in the crabs then bring them back to the boil and cook for 2 minutes. Strain and let the crabs cool a little, then chop with a large knife.
Melt the butter in a heavy-based pan and add the chopped onion, carrot, celery and the bay leaf. Cook without browning. Stir once or twice then add the crab. Stir, then add the cognac. Allow to boil off then add the tomatoes, tomato purée, wine, tarragon and stock. Bring to the boil and simmer for 30 minutes.
Remove the tough claw shells from the soup before liquidizing in a liquidizer or food processor in two or three batches. Process in short bursts until the shell is broken into small pieces about the size of your finger nail. Avoid producing puréed shell, the aim is to extract all possible flavour from any meat left sticking to the shell, particularly in the body section, rather than to extract flavour from the shell itself. Strain the soup through a conical strainer pushing as much liquid through as you can with the back of a ladle to extract all the juices.
Then, pass the soup through a fine strainer before returning to the heat. Bring to the boil, add the cream then season with cayenne pepper, lemon juice, salt and black pepper. Reduce the volume by simmering if you think the flavour needs concentrating.