Freeze the crabs for 2 hours to immobilise them, then drop them into boiling water and cook just until the shells turn red. Remove with tongs and set aside until cool enough to handle.
Following steps 3–5 on, cut the crab and pick out the meat from the body. Reserve four crab claws and pick the meat out of the rest. Reserve the meat and shells separately.
Heat the butter in a large saucepan. Add the vegetables, bay leaf and thyme and cook over medium heat for 3 minutes, without allowing the vegetables to colour. Add the reserved crab shells, the tomato paste, brandy and wine and simmer for 2 minutes, or until reduced by half.
Add the stock and 500ml (17fl oz/2cups) water and bring to the boil. Reduce the heat and simmer for 5 minutes. Remove the shells and finely crush them with a mortar and pestle (or in a food processor with alittle of the stock). Return the crushed shells to the soup with the rice and the reserved crabmeat. Bring to the boil, reduce the heat, cover and simmer for about 20 minutes, or until the rice is soft.
Immediately strain the bisque into a clean saucepan through a fine sieve lined with damp muslin, pressing down firmly on the solids to extract all the soup. Add the cream and season with salt and cayenne pepper, then gently reheat. Ladle into warmed soup bowls and garnish with the crab claws.