In the 18th century, Catholic monks at the Jerónimos Monastery in Belém monks used egg whites for starching nuns' habits and there garments. This left many surplus egg yolks. To ovoid this waste, they cleverly used them to make unique custard tarlets. They sold their closely guarded secret to the Pastéis de Belém bakery, where it is still closely guarded, with only six people knowing it at a time.