In a bowl, whisk flour, salt, and sugar. Cut in cold butter until pea-sized crumbs form, then add ice water until dough just comes together.
Food processor option: Pulse flour, salt, sugar, and butter until crumbly, then add water just until clumps form.
Divide dough into two discs, wrap, and chill for 30 minutes.
Stir together brown sugar, flour, cinnamon, and melted butter for the filling.
Roll one disc into a rectangle and place on a parchment-lined baking sheet. Spread filling evenly, leaving a 2 cm border. Roll out second disc and lay on top. Crimp edges with a fork to seal.
Prick the top all over with a fork. Bake at 375 F for 25–30 minutes, until golden brown.
Whisk powdered sugar, milk, and cinnamon into a glaze. Spread or drizzle over pastry.
Let set for 10 minutes before slicing into squares.
