Make the Pastry: Whisk flour, sugar, and salt in a large bowl. Cut in butter until pea-sized crumbs form. Gradually add ice water, stirring until the dough just comes together. Divide into 2 discs, wrap, and refrigerate for at least 2 hours.

Roll the Dough: Roll one chilled disc into a 9×12-inch rectangle, about ⅛ inch thick. Trim edges, then cut into 9 rectangles (3×4 inches). Transfer to a parchment-lined baking sheet and chill. Repeat with the second disc for the tops.

Mix the Filling: Combine brown sugar, cinnamon, and flour in a small bowl.

Assemble the Pop Tarts: Brush egg wash over the bottoms. Spoon 1 heaping tablespoon of filling onto each, leaving a ¼-inch border. Brush tops with egg wash, then place egg-wash-side-down over the filling. Press edges to seal and crimp with a fork.

Vent & Chill: Poke holes in tops to allow steam to escape. Chill for 20 minutes while preheating oven to 350°F (177°C).

Bake: Brush tops with remaining egg wash and bake for 25–30 minutes, until golden brown. Cool completely on a wire rack.

Glaze: Whisk confectioners’ sugar, milk, cinnamon, and vanilla until smooth. Spread over cooled pop tarts and let set for 1 hour before serving.