**Prepare the Pastry Dough:** In a large mixing bowl, combine the flour, sugar, and salt. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add ice water, one tablespoon at a time, mixing until the dough comes together. Divide the dough in half, shape into discs, wrap in plastic wrap, and refrigerate for at least 1 hour.

**Make the Filling:** In a small bowl, mix brown sugar, cinnamon, and flour until well combined.

**Roll Out the Dough:** On a floured surface, roll out one disc of dough into a 1/8-inch thick rectangle. Cut into 3x4-inch rectangles. Repeat with the second disc of dough. You should have about 16 rectangles.

**Assemble the Pop Tarts:** Place half of the rectangles on a parchment-lined baking sheet. Brush the edges with egg wash. Add a heaping tablespoon of filling to the center of each rectangle, then top with another rectangle of dough. Press the edges together with a fork to seal. Prick the tops with a fork to create vent holes.

**Chill and Preheat Oven:** Refrigerate the assembled pop tarts for 20 minutes while preheating the oven to 375°F (190°C).

**Bake:** Brush the tops with egg wash and bake for 20–25 minutes or until golden brown. Let cool completely.

**Prepare the Frosting:** In a small bowl, whisk together powdered sugar, cinnamon, milk, and vanilla extract until smooth. Spread over the cooled pop tarts and let the frosting set before serving.