Make the filling: Open dates and remove pits. Dice. Boil and reduce to simmer. While simmering, mash the dates until you’re happy. I liked mine with some chunks left in them. If you don’t like any chunks at all, you might have to put it in the blender. When dates look thickened enough to spread with a spoon, they’re done.
2 cups (8 1/2 ounces) all-purpose flour
1 tablespoon sugar
1 teaspoon salt
1 cup (2 sticks or 8 ounces) unsalted butter, cut into pats
1 large egg
2 tablespoons (1 ounce) milk
1 additional large egg (to brush on pastry)
Make the dough: Whisk together the flour, sugar, and salt. Work in the butter with your fingers, pastry blender or food processor until pea-sized lumps of butter are still visible, and the mixture holds together when you squeeze it. If you’ve used a food processor, transfer the mixture to a large bowl. Whisk the first egg and milk together and stir them into the dough, mixing just until everything is cohesive, kneading briefly on a well-floured counter if necessary.
Divide the dough in half (approximately 8 1/4 ounces each), shape each half into a smooth rectangle, about 3×5 inches. You can roll this out immediately (see Warm Kitchen note below) or wrap each half in plastic and refrigerate for up to 2 days.
Assemble the tarts: If the dough has been chilled, remove it from the refrigerator and allow it to soften and become workable, about 15 to 30 minutes. Place one piece on a lightly floured work surface, and roll it into a rectangle about 1/8″ thick, large enough that you can trim it to an even 9″ x 12″. [You can use a 9" x 13" pan, laid on top, as guidance.] Repeat with the second piece of dough. Set trimmings aside. Cut each piece of dough into thirds – you’ll form nine 3″ x 4″ rectangles.
Beat the additional egg and brush it over the entire surface of the first dough. This will be the “inside” of the tart; the egg is to help glue the lid on. Place a heaping tablespoon of filling into the center of each rectangle, keeping a bare 1/2-inch perimeter around it. Place a second rectangle of dough atop the first, using your fingertips to press firmly around the pocket of filling, sealing the dough well on all sides. Press the tines of a fork all around the edge of the rectangle. Paint with egg whites. Repeat with remaining tarts.
Gently place the tarts on a lightly greased or parchment-lined baking sheet. Prick the top of each tart multiple times with a fork; you want to make sure steam can escape, or the tarts will become billowy pillows rather than flat toaster pastries. Refrigerate the tarts (they don’t need to be covered) for 30 minutes, while you preheat your oven to 350°F.
Bake the tarts: Remove the tarts form the fridge, and bake them for 20 to 25 minutes, until they’re a light golden brown. Cool in pan on rack.
Note: I adapted this recipe from http://smittenkitchen.com/2010/04/homemade-pop-tarts/ where there are ideas for other fillings. Apparently this recipe had there been adapted already from someone else's site.