There are two kinds of Kings Cakes in France; one is layers of puff pastry filled with almond cream, and the other is a circular yeasted cake (galette), more popular in the south of France, festooned with sugar chunks. It is tradition to bake a fève (bean) into the tart and whoever gets the bean gets to be the roi (king) for the day and wear a gilded paper crown that bakeries often give with the galettes. The cake is used to “call the kings” to Epiphany.
There is a similar tradition in the southern U.S., also called a King Cake, during Mardi Gras. This cake is more like the round galette and a baby trinket is hidden inside.
