For the dough:
- 9 whole graham crackers (1 sleeve)
- 1 1/4 cups all-purpose flour
- 1 teaspoon fine salt
- 2 sticks (8 ounces) cold unsalted butter, cut into 1/2-inch cubes
- 2 large egg yolks
- 1/4 cup whole milk
- Flour, for rolling the dough
- 1 large egg
- 1 teaspoon water
- About 1 cup marshmallow fluff
For the topping:
- 6 ounces milk chocolate, finely chopped
For the dough:
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).
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 the butter pieces are the size of peas.
Whisk the egg yolks and milk in a measuring cup or 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-by-5-inch) rectangles. Wrap each tightly in plastic wrap and refrigerate for at least 1 1/2 hours.
To assemble the tarts:
Heat the oven to 375°F. Line a baking sheet with parchment paper; set aside.
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.
Whisk the egg and water in a small bowl until evenly combined; set aside. Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
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 fluff onto each of these rectangles, leaving a 3/4-inch border. Place 3 nonchilled rectangles over the marshmallow fluff, 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 fluff. 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.
When the dough is ready, bake until golden brown and the marshmallow fluff has puffed, about 20 to 23 minutes (the marshmallow fluff will deflate as it cools). Transfer to a wire rack and let cool completely.
For the topping:
While the tarts are cooling, place the milk chocolate in a small microwave-safe bowl. Melt the chocolate in 15-second bursts in the microwave, stirring after each burst, until smooth.
Set the wire rack with the tarts on it over a baking sheet. Place melted chocolate in a zip-top bag, cut a small opening on one corner of the bag, and drizzle chocolate over tarts. Eat right away, or let the chocolate set before serving.