01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large mixing bowl, beat softened butter and sugar together until light and fluffy, about 2 minutes. Add flour and salt, mixing until a crumbly dough comes together.
03 - Press dough evenly into the bottom of the prepared pan. Bake for 15 minutes until the edges are lightly golden. Remove from oven and set aside.
04 - While the crust bakes, combine diced peaches, sugar, cornstarch, and lemon juice in a medium bowl. Toss gently until peaches are evenly coated and cornstarch is fully dissolved.
05 - In a separate bowl, whisk together flour, brown sugar, cinnamon, and salt. Cut in cold cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs with some larger pieces remaining.
06 - Spread the peach filling evenly over the warm pre-baked crust. Sprinkle the streusel topping uniformly over the peaches, covering the entire surface.
07 - Return pan to oven and bake for 30 minutes, or until the streusel is deeply golden and the peach filling is bubbling around the edges.
08 - Allow bars to cool completely in the pan on a wire rack. Once fully set, lift the parchment overhang to remove the slab, then slice into 12 even bars.