From the Middle Ages, the epiphany has been celebrated with a special Twelfth Night cake : la galette des rois, literally the King's cake.
The cake contains a lucky charm (une fève) which originally was a bean, a symbol of fertility. Whoever found the charm in their slice of cake, became King or Queen and had to buy a round of drinks for all their companions. The charm can take any shape or form and can either be very plain or more sophisticated (glazed or handpainted). It sometimes represents a religious figure such as the baby Jesus, but it can be virtually anything. Although nowadays very often made of plastic, old-fashioned china charms are still used and they have become a collectable item.
The modern Galette des Rois is made of puff pastry and can be plain or filled with frangipane, an almond-flavoured paste. It is sold in all French bakeries and eating the galette at the beginning of January is still a very popular tradition and an opportunity for families and friends to gather around the table. The youngest person in the room (usually a child) hides under the table and shouts out which guest each slice of cake should be given to. The person who finds the fève in their slice of galette becomes the King or Queen and is given a golden paper crown. The King or Queen then has to choose his Queen or her King, by dropping the lucky charm in their glass.

(From the askoxford.com website).