Use a high-quality commercial all-butter puff pastry, such as Dufour, which is available in the freezer section of specialty food markets. Or purchase puff pastry dough from your local French bakery. Preheat the oven to 425°. Line 2 baking sheets with parchment paper. Lightly roll out the puff pastry 1/8 inch thick if using 14 ounces of dough or 1/4 inch thick if using 16 ounces of dough. Cut the pastry into sixteen 3 1/2-by-4 1/2-inch rectangles. Place a piece of chocolate in the center of each rectangle and fold the long sides over the chocolate like a business letter. Seal the exposed edge with some of the beaten egg. Repeat with the remaining dough rectangles, chocolate and egg wash. Arrange 8 pastries on each baking sheet. Refrigerate 1 sheet of pastries and bake the other in the center of the oven for 10 minutes. Reduce the heat to 375° and bake for about 15 minutes longer, or until golden brown and cooked through. Transfer the pains au chocolat to a rack to cool slightly. Repeat with the remaining pastries. Serve slightly warm.