This is the same recipe as all butter pie crust. See step-by-step photos in the pie crust post if you need visuals for making the pastry. I usually make the pastry the night before. In a large bowl, whisk together the flour, sugar, and salt. Using a pastry cutter or two forks, cut the butter into the mixture until it resembles a coarse meal (pea-sized bits with a few larger bits of fat is okay). You can also use a food processor, and pulse the mixture together, but be careful not to overwork the ingredients. Drizzle the water over the flour mixture, 1 Tablespoon at a time, and stir with a spatula after every Tablespoon has been added until the dough begins to form large clumps. Do not add any more water than needed.

Transfer the dough to a floured work surface. Using floured hands, gently bring the dough mixture together into a ball. Avoid overworking the dough. If it feels too dry or is too crumbly to form a ball, dip your fingers in cold water and then continue bringing dough together. If it feels too sticky, sprinkle more flour on the dough and then continue bringing it together. Using a sharp knife or bench scraper, cut the dough in half. Gently flatten each half into 1-inch-thick discs. Wrap each tightly in plastic wrap. Refrigerate for at least 2 hours and up to 5 days (or in the freezer for up to 3 months).

Remove 1 chilled dough disc from the refrigerator and allow it to sit at room temperature for 5 minutes. Keep the other disc in the refrigerator. After 5 minutes, place disc onto a lightly floured work surface, and roll it into a 9×12-inch rectangle, about 1/8 inch thick. Trim the sides as needed. With a pizza cutter or sharp knife, cut the dough into thirds and each third into thirds again. You will end up with 9 rectangles, each measuring 3×4 inches. Use a ruler to help make this easier and more accurate. Place each of the 9 rectangles onto a baking sheet lined with parchment paper or a silicone baking mat. The pop tarts will not spread much in the oven, so you may place them near one another. Place the baking sheet in the refrigerator.

Repeat step 3 with the 2nd disc of dough to make 9 more rectangles. Place each of the 9 rectangles onto a baking sheet lined with parchment paper or a silicone baking mat. Place the baking sheet in the refrigerator.

Mix the brown sugar, cinnamon, and flour together in a small bowl.

Remove 1 baking sheet of pastry rectangles from the refrigerator. These will be the bottoms of your pop tarts. Brush egg wash over the entire surface of each rectangle. Place a heaping Tablespoon of the filling into the center of each rectangle and spread it around with the back of a spoon, leaving a 1/4-inch border around the edges.

Remove the 2nd baking sheet of pastry rectangles from the refrigerator. These will be the tops of your pop tarts. Brush egg wash over the entire surface of each top rectangle, then place each top rectangle, egg-wash-side-down, on the filling-topped bottoms. Use your fingertips to press firmly around the pocket of filling, sealing the dough well on all sides. Reserve remaining egg wash.

Using a toothpick, poke 6 to 8 holes in the tops of each filled pastry to allow the steam to escape. You can, instead, use a knife to make a few small slits. Seal the edges tightly by crimping with a fork. Refrigerate the filled pop tarts, uncovered, for at least 20 minutes and up to 1 hour.

Preheat the oven to 350°F (177°C). Once the unbaked pop tarts have chilled for 20 minutes, remove from the refrigerator and brush the tops with the remaining egg wash. Bake for 25–30 minutes or until golden brown, rotating the pan halfway through baking. Let the pop tarts cool on the pan for about 5 minutes, then transfer to a cooling rack to cool completely before icing.

In a medium bowl, whisk together the confectioners’ sugar, milk, cinnamon, and vanilla. You want a thick glaze, but not too thick that it is hard to spread. If it’s too thick, add more milk, 1 teaspoon at a time, until it reaches a spreadable consistency. Spoon the icing on top of each cooled pop tart, and use the back of the spoon or an icing spatula to spread. The icing will set in about 1 hour.

Store pop tarts in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days. To reheat, bake in a 350°F (177°C) oven for 10 minutes.