01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk flour, salt, and baking powder. Add cold butter and cut it in using a pastry cutter or fork until the mixture forms coarse crumbs.
03 - Add brown sugar, egg, and vanilla extract to the bowl. Mix until a soft dough is formed, being careful not to overmix.
04 - Press two-thirds of the dough evenly into the base of the prepared pan.
05 - In a separate bowl, combine brown sugar, cinnamon, flour, salt, and melted butter until a thick paste is formed. Spread this filling evenly over the pressed crust.
06 - Crumble the remaining dough evenly over the filling and gently press to adhere.
07 - Bake for 28 to 32 minutes, or until the edges are golden and the center is set. Allow the bars to cool completely in the pan.
08 - In a small bowl, whisk together powdered sugar, milk, vanilla extract, and cinnamon until smooth.
09 - Drizzle glaze over the cooled bars. Let the glaze set for 20 minutes before slicing and serving.