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.

Drizzle the water over the flour mixture, 1 Tablespoon at a time, and stir with a spatula until the dough begins to form large clumps.

Transfer the dough to a floured work surface. Using floured hands, gently bring the dough mixture together into a ball. Wrap each tightly in plastic wrap and refrigerate for at least 2 hours.

Remove 1 chilled dough disc from the refrigerator and allow it to sit at room temperature for 5 minutes. Roll it into a 9×12-inch rectangle, about ⅛ inch thick. Cut the dough into thirds and each third into thirds again to make 9 rectangles.

Place each of the 9 rectangles onto a baking sheet lined with parchment paper. Repeat with the 2nd disc of dough.

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

Brush egg wash over the entire surface of each rectangle. Place a heaping Tablespoon of the filling into the center of each rectangle.

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.

Using a toothpick, poke 6 to 8 holes in the tops of each filled pastry. Seal the edges tightly by crimping with a fork. Refrigerate the filled pop tarts for at least 20 minutes.

Preheat the oven to 350°F (177°C). Brush the tops with the remaining egg wash and bake for 25–30 minutes or until golden brown.

In a medium bowl, whisk together the confectioners’ sugar, milk, cinnamon, and vanilla. Spoon the icing on top of each cooled pop tart.

Store pop tarts in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.