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1
Whisk the flour, cocoa, sugar, and salt in a large bowl until combined.
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2
Add the butter and toss with your fingers until well coated in the flour mixture.
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3
Using a pastry blender or your fingers, cut or rub the butter into the dry ingredients until reduced to pea-size pieces.
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4
Whisk the egg yolks and milk in a small bowl until combined.
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5
Add the egg-milk mixture to the flour mixture and mix with your hands until large clumps form.
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6
Turn the mixture out onto a work surface and knead briefly, smearing the butter into the dough with the heel of your palm until the dough completely comes together, about 1 minute.
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7
Divide the dough into 2 equal portions and shape into 2 (6-by-5-inch) rectangles.
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8
Wrap each tightly in plastic wrap and refrigerate for at least 1 hour.
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9
Meanwhile, make the filling.
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10
Place the chocolate and butter in a medium heatproof bowl; set aside.
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11
Combine the cream, sugar, and salt in a small saucepan over medium heat and stir until the sugar dissolves and the mixture is just at a simmer, about 4 minutes.
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12
Pour the cream mixture over the chocolate and butter and let sit until melted, about 4 minutes.
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13
Gently stir until smooth.
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14
Refrigerate until stiffened but still spreadable, about 15 minutes.
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15
Remove from the refrigerator and let sit at room temperature until youre ready to assemble the tarts.
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16
Heat the oven to 375 degrees F. Line a baking sheet with parchment paper; set aside.
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17
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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18
Using a pizza cutter or sharp knife, trim the dough to a 10-1/2-by-9-inch rectangle.
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19
Cut that into 6 equal rectangles (each about 3 1/2 inches wide by 4 1/2 inches tall).
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20
Using a flat spatula, transfer the rectangles to the prepared baking sheet, leaving about 2 inches of space between each.
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21
Place the baking sheet in the refrigerator.
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22
Whisk the egg and water in a small bowl until evenly combined; set aside.
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23
Roll out the second dough portion to the same dimensions as the first, trim, and cut into 6 rectangles.
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24
Using a fork, prick the dough all over.
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25
Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over each dough rectangle.
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26
Spoon 1 tablespoon plus 1 teaspoon of the chocolate filling onto each rectangle.
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27
Spread the filling into an even layer, leaving a 3/4-inch border.
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28
Place the pricked rectangles on top of the chocolate-covered rectangles.
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29
Press on the edges to adhere, and push down gently on the filling to slightly flatten.
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30
Using a fork dipped in flour, crimp the edges of the tarts.
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31
Bake until puffed and flaky, about 23 to 25 minutes.
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32
Transfer to a wire rack and let cool completely before glazing.
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33
Place all the ingredients in a small bowl and whisk until evenly combined.
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34
(You may need to add more milk by the 1/2 teaspoon if the glaze is too thick.)
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35
Set the wire rack with the tarts on it over a baking sheet.
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36
Using a spoon, drizzle about 1 tablespoon of the glaze over each pastry.
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37
Let set before eating, about 15 minutes.
