For the dough:

Cup of Yum
1. Place the graham crackers in a resealable plastic bag and smash them with a rolling pin until you have fine crumbs. Alternately, blitz them in a food processor.
2. 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.
3. Whisk the egg yolks and milk in a small bowl until combined. Add the egg and milk mixture to the flour mixture and mix with your hands until a dough forms (it will be a little sticky).
4. 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.

To assemble the tarts:

1. Heat the oven to 375°F. Line a baking sheet with parchment paper; set aside.
2. 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.
3. Trim the dough to a 10-1/2 by 9 inch rectangle with a pizza cutter or sharp knife and cut into 6 rectangles. Transfer each piece of dough to the prepared pan with a spatula, leaving about 2 inches of space between each. Place in the refrigerator.
4. Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
5. Whisk together the egg and water in a small bowl to form an egg wash.
6. Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over the dough rectangles.
7. 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.
8. 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.
9. Return to the refrigerator for 15 minutes.
10. 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.
11. To make the topping, melt the chocolate chips in short bursts in the microwave, stirring every 15 seconds until smooth and melted.
12. Transfer the melted chocolate to a piping bag. Drizzle the chocolate lightly over the poptarts.
13. Eat immediately or let the chocolate set before serving.