Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
In a large bowl, combine flour, sugar, and salt. Cut in butter until mixture resembles coarse crumbs.
Add ice water, one tablespoon at a time, until the dough comes together.
Divide dough in half, shape into discs, wrap in plastic, and chill for 30 minutes.
In a small bowl, mix brown sugar, cinnamon, and flour for the filling.
Roll out dough on a floured surface to ⅛-inch thickness. Cut into rectangles.
Spoon filling onto half of the rectangles, leaving a border around the edges.
Brush edges with egg wash and place another rectangle of dough on top. Press edges with a fork to seal.
Transfer to baking sheet, brush tops with egg wash, and bake for 18-20 minutes until golden brown.
Let cool before adding optional icing.