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1
Place the graham crackers in a resealable plastic bag, press out the air, and seal, leaving about 1 inch unsealed.
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2
Using a rolling pin, roll over the crackers until they are fine, uniform crumbs (you should have about 1 1/4 cups).
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3
Whisk the flour, graham cracker crumbs, and salt in a large bowl until combined.
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4
Add the butter and toss with your fingers until well coated in the flour mixture.
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5
Using a pastry blender or your fingers, cut the butter into the dry ingredients until reduced to pea-size pieces.
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6
Whisk the egg yolks and milk in a small bowl until combined.
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7
Add the egg-milk mixture to the flour mixture and mix with your hands until a dough forms (it will be a little sticky).
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8
Divide the dough into 2 equal portions and shape into 2 (6-by-5-inch) rectangles.
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9
Wrap each tightly in plastic wrap and refrigerate for at least 1 1/2 hours.
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10
Heat the oven to 375 degrees F. Line a baking sheet with parchment paper; set aside.
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11
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.
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12
Using a pizza cutter or sharp knife, trim the dough to a 10-1/2-by-9-inch rectangle.
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13
Cut that into 6 equal rectangles (each about 3 1/2 inches wide by 4 1/2 inches tall).
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14
Using a flat spatula, transfer the rectangles to the prepared baking sheet, leaving about 2 inches of space between each.
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15
Place the baking sheet in the refrigerator.
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16
Whisk the egg and water in a small bowl until evenly combined; set aside.
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17
Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
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18
Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over 3 of the dough rectangles.
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19
Spread 2 rounded tablespoons of marshmallow creme onto each of these rectangles, leaving a 3/4-inch border.
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20
Place 3 nonchilled rectangles over the marshmallow creme, press on the edges to adhere, and push down gently on the filling to slightly flatten.
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21
Using a fork dipped in flour, crimp the edges of the tarts.
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22
Repeat with the remaining dough rectangles and creme.
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23
Cut two 2-inch slits to form an X in the center of each tart.
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24
Place in the refrigerator until the dough has stiffened slightly, about 15 to 20 minutes.
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25
When the dough is ready, bake until golden brown and the marshmallow creme has puffed, about 20 to 23 minutes (the marshmallow creme will deflate as it cools).
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26
Transfer to a wire rack and let cool completely.
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27
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.
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28
Place the chocolate in a dry, heatproof bowl large enough to sit over the water without touching it.
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29
Set the bowl over the saucepan and stir until the chocolate is completely melted, about 5 minutes.
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30
Set the wire rack with the tarts on it over a baking sheet.
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31
Using a spoon, drizzle each tart with melted chocolate.
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32
Eat right away, or let the chocolate set before serving.
