A custard made with beasting, or beesting - the first milk drawn from a cow after she gives birth, it is unusually rich (White 1932, etc).
'The Medical Times' in 1857 has; "In some country districts, such milk is considered a treat, and the farmer, when his cow calves, sends round to his neighbours a basin of "beastings" for the express manufacture of a "beastings pudding." Our Cockneys, accustomed to the refinements of St. Paul's Churchyard, turn up their noses."