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 or wrap each half in plastic and refrigerate for up to 2 days. I would recommend refrigerating it, however. Pastry dough needs to stay cool in order to produces a flaky, buttery crust. I wouldn't risk it, so making the dough a day before is probably the best idea.
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 (or fourths to make minis). I decided to make the minis because the regulars seemed too big. Also, we have like 20 people on the team so if I just brought 9 of anything, people would riot.
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 half a tablespoon (full if making the regular size) of filling into the center of each rectangle, keeping a bare 1/2-inch perimeter around it. See below on how to make the fillings. 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. 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. Brush the tops with the remaining egg wash and sprinkle with sugar. This will give the pop tarts a nice shiny color. Refrigerate the tarts (they don’t need to be covered) for 30 minutes, while you preheat your oven to 350°F.
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.