Blend the beans in a food processor until they form a smooth paste. Add the butter, vinegar and bay leaf. Season with salt and pepper. Blend for 2 minutes. Separate the mixture into 12 portions shaped like fingers.
Beat the egg with some water, and put in a shallow dish. Put the breadcrumbs in another shallow dish.
Coat the fingers, or croquettes, first with the egg, then with the breadcrumbs. Repeat the egg and breadcrumbs process to ensure that the croquettes are well coated. Chill for 1 hour.
Heat enough oil for deep-frying in a heavy frying pan over a mediumhigh heat. When the oil is hot enough, deep-fry the croquettes until golden brown. Remove from the pan using a spatula or slotted spoon, and drain on kitchen paper. Serve hot.