Roll out your croissant dough, but instead of cutting triangles keep the dough in rectangles. Place a piece of chocolate about 1/4 of the way back from the leading edge.
Using the parchment paper to support your croissant dough, carefully flip the leading edge over the chocolate.
Keep rolling the dough around the chocolate gently until the whole think is rolled up.
Let the pain au chocolat rise like you would a croissant. Brush with egg wash and cook with your other croissants as directed.