Break up the crab shells using the back of a large knife and a mallet.
Heat 2 tablespoons of the oil in a large saucepan. Add the onion, carrots, celery and bay leaf and fry until soft but not coloured. Add the broken shells and fry for 2–3 minutes, then add the Cognac, tomatoes and tomato purée.
Pour in the stock and bring to the boil, then reduce the heat and simmer for 30 minutes.
Heat the remaining oil in a frying pan and add the garlic. Fry for 1 minute to flavour the oil, then remove and discard the garlic. Add the bread cubes to the garlic oil and fry until golden brown.
Remove the claw shell from the pan. Place the rest of the shells and liquid in a liquidizer or food processor in batches and blend until the shells are in pieces of about 1 cm (½ inch). Pass the liquid and shells through a fine sieve lined with a piece of muslin.
Pour the liquid back into a clean saucepan and bring to the boil, then add the cream and cayenne. If the flavour needs intensifying, simmer the soup to reduce it. Season with salt and pepper and serve in bowls topped with the garlic croutons.