S'mores Pop Tarts

1. 1
   Place the graham crackers in a resealable plastic bag, press out the air, and seal, leaving about 1 inch unsealed. Using a rolling pin, roll over the crackers until they are fine, uniform crumbs (you should have about 1 1/4 cups).
2. 2
   Whisk the flour, graham cracker crumbs, and salt in a large bowl until combined. Add the butter and toss with your fingers until well coated in the flour mixture. Using a pastry blender or your fingers, cut the butter into the dry ingredients until reduced to pea-size pieces.
3. 3
   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).
4. 4
   Divide the dough into 2 equal portions and shape into 2 (6-by-5-inch) rectangles. Wrap each tightly in plastic wrap and refrigerate for at least 1 1/2 hours.

To assemble the tarts:

1. 1
   Heat the oven to 375°F. Line a baking sheet with parchment paper; set aside.
2. 2
   Lightly dust a 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. Using a pizza cutter or sharp knife, trim the dough to a 10-1/2-by-9-inch rectangle. Cut that into 6 equal rectangles (each about 3 1/2 inches wide by 4 1/2 inches tall). Using a flat spatula, transfer the rectangles to the prepared baking sheet, leaving about 2 inches of space between each. Place the baking sheet in the refrigerator.
3. 3
   Whisk the egg and water in a small bowl until evenly combined; set aside.
4. 4
   Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
5. 5
   Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over 3 of the dough rectangles. Spread 2 rounded tablespoons of marshmallow crème onto each of these rectangles, leaving a 3/4-inch border. Place 3 nonchilled rectangles over the marshmallow crème, press on the edges to adhere, and push down gently on the filling to slightly flatten. Using a fork dipped in flour, crimp the edges of the tarts. Repeat with the remaining dough rectangles and crème. Cut two 2-inch slits to form an “X” in the center of each tart. Place in the refrigerator until the dough has stiffened slightly, about 15 to 20 minutes.
6. 6
   When the dough is ready, bake until golden brown and the marshmallow crème has puffed, about 20 to 23 minutes (the marshmallow crème will deflate as it cools). Transfer to a wire rack and let cool completely.

For the topping:

1. 1
   While the tarts are cooling, bring a small saucepan filled with about 1 inch of water to a simmer over high heat; once simmering, turn off the heat. Place the chocolate in a dry, heatproof bowl large enough to sit over the water without touching it. Set the bowl over the saucepan and stir until the chocolate is completely melted, about 5 minutes.
2. 2
   Set the wire rack with the tarts on it over a baking sheet. Using a spoon, drizzle each tart with melted chocolate. Eat right away, or let the chocolate set before serving.