Place the graham crackers in a resealable plastic bag and smash them with a rolling pin until you have fine crumbs.

Whisk the flour, graham cracker crumbs, and salt in a large bowl until combined. Add the butter and, using a pastry blender or your fingers, cut the butter into the dry ingredients until you have pea-sized pieces.

Whisk the egg yolks and milk in a small bowl until combined. Add the egg-milk mixture to the flour mixture and mix with your hands until a dough forms (it will be a little sticky).

Divide the dough into 2 equal portions and shape into 2 6 x 5-inch rectangles. Wrap each tightly in plastic wrap and refrigerate for at least 1 1/2 hours.

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.