The oval-shaped Mardi Gras cake containing a small plastic baby is king cake.
In Louisiana and other parts of the Gulf Coast, king cake is a Carnival-season pastry traditionally eaten from Epiphany through Mardi Gras. It is often formed as a ring or oval of braided, brioche-like dough, then covered with icing and purple, green, and gold sugar. Modern versions may contain cinnamon, cream cheese, praline, or fruit fillings.
The hidden baby represents the infant Jesus and connects the cake to the older European tradition of Epiphany cakes. Finding the baby is considered a sign of luck and prosperity. In many celebrations, the person who finds it is expected to provide the next cake or host a future Mardi Gras gathering.
The American Mardi Gras tradition became especially associated with New Orleans during the 20th century. The purple, green, and gold color scheme has been standard there since 1872, with purple representing justice, green faith, and gold power. The baby is sometimes supplied separately because placing a plastic object inside baked food can create a choking or liability concern.