Game plan: You can roll the dough scraps into 1 or 2 more tarts, but you’ll then need extra filling and glaze.
You can make the dough a day in advance; just be sure to let it soften at room temperature for about 10 minutes before rolling out.
Though best when eaten fresh, the cooled pop tarts can be stored in an airtight container for up to 2 days at room temperature. Reheat them, if desired, in a 300°F oven on a baking sheet until hot, about 10 minutes. Freezing is not recommended.
Whisk the flour, sugar, cinnamon, 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.
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 large clumps form. 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.
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 hour. Meanwhile, make the filling.
For the brown sugar–cinnamon filling:
Place all the ingredients in a small bowl and whisk until evenly combined; set aside.
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. Using a fork, prick the dough all over.
Remove the baking sheet from the refrigerator and brush a thin coating of the egg wash over each dough rectangle. Divide the brown sugar–cinnamon filling among the rectangles. Spread the filling into an even layer, leaving a 3/4-inch border.
Place the pricked rectangles on top of the brown sugar–cinnamon-covered rectangles. 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. Bake until golden brown, about 23 to 25 minutes. Transfer to a wire rack and let cool completely before glazing.
For the glaze:
Place all the ingredients in a small bowl and whisk until evenly combined. (You may need to add more milk by the 1/2 teaspoon if the glaze is too thick.)
Set the wire rack with the tarts on it over a baking sheet. Using a spoon, drizzle about 1 tablespoon of the glaze over each tart. Let set before eating, about 15 minutes.