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, cream together the softened butter and sugar using an electric mixer until light and fluffy, about 2 minutes. Add the flour and salt, mixing on low speed until a crumbly dough forms.
03 - Press the dough evenly into the bottom of the prepared pan. Bake for 15 minutes until the edges are lightly golden. Remove from the oven and set aside.
04 - While the crust bakes, combine the diced peaches, sugar, cornstarch, and lemon juice in a medium bowl. Toss gently until the peaches are evenly coated and the cornstarch is fully dissolved.
05 - In a separate bowl, whisk together the flour, brown sugar, cinnamon, and salt. Add the cold cubed butter and cut it in 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 par-baked crust. Sprinkle the cinnamon streusel topping uniformly over the peach layer, covering as much surface area as possible.
07 - Return the pan to the oven and bake for an additional 30 minutes, or until the streusel topping is golden brown and the peach filling is bubbling around the edges.
08 - Allow the bars to cool completely in the pan on a wire rack. Once fully cooled, lift the slab out using the parchment overhang, slice into 12 even bars, and serve.