S'mores Pop Tarts

1. 1Place 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. 2Whisk 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. 3Whisk 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. 4Divide 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. 1Heat the oven to 375°F. Line a baking sheet with parchment paper; set aside.
2. 2Lightly 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. 3Whisk the egg and water in a small bowl until evenly combined; set aside.
4. 4Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
5. 5Remove 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. 6When 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. 1While 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. 2Set 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.