Heat the oven to 375°F. Line a baking sheet with parchment paper; set aside.
Lightly dust your work surface with flour and roll 1 dough portion out into a rough 12-by-10-inch rectangle, rotating the dough and reflouring the surface and rolling pin often to prevent the dough from sticking.
Trim the dough to a 10-1/2 by 9 inch rectangle with a pizza cutter or sharp knife.
Cut that into 6 equal rectangles.
Transfer the rectangles to the prepared pan with a spatula, leaving about 2 inches between each rectangle of dough. Place the baking sheet in the refrigerator.
Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
Whisk together the egg and water in a small bowl to form an egg wash.
Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over the dough rectangles.
Break the chocolate bars into 6 small pieces and place a piece in the center of each of the chilled dough rectangles.
Drop about 2 tablespoons of marshmallow cream over the chocolate bar pieces.
Top with the un-chilled dough rectangles and press down slightly to seal the edges. Use a fork to crimp the edges.
Cut a small X in the top of each poptart to vent.
Return to the refrigerator for 15 minutes.
Bake until golden brown and the marshmallow cream has puffed up, about 20-25 minutes. Transfer to a wire rack to cool. The marshmallow cream will deflate as it cools.
To make the topping, melt the chocolate chips in a small sauce pan over very low heat or in the microwave.
Transfer the melted chocolate to a piping bag (or just a plastic bag) and snip off the corner of the bag to drizzle on the chocolate.
Eat immediately or let the chocolate set before serving.