Pain au chocolat (French: lit. 'bread with chocolate'), also known as chocolatine in the south-west part of France and in French-speaking parts of Canada, couque au chocolat in Belgium, or chocolate croissant in the United States, is a type of Viennoiserie pastry consisting of a cuboid-shaped piece of yeast-leavened laminated dough, similar in texture to a puff pastry, with one or two pieces of dark chocolate in the center. The chocolate usually has a slight bite to the texture.
Pain au chocolat is made of the same layered doughs as a croissant. Often sold still hot or warm from the oven, they are commonly sold alongside croissants in French bakeries and supermarkets.
Read more on Wikipedia.You can also make pain au chocolat at home with a good recipe and some time and patience.