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 the softened butter and granulated sugar together until light and fluffy, about 2 minutes. Add the flour and salt, then mix until a crumbly dough comes together.
03 - Press the dough evenly into the bottom of the prepared pan. Bake for 15 minutes until the crust is lightly golden. Remove from the oven and set aside.
04 - While the crust bakes, combine the diced peaches, granulated sugar, cornstarch, and lemon juice in a medium bowl. Toss gently until the peaches are evenly coated.
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 forms large, pea-sized crumbs.
06 - Spread the peach filling evenly over the warm pre-baked crust. Scatter the streusel topping uniformly over the peach layer.
07 - Return the pan to the oven and bake for 30 minutes, or until the streusel is deep golden brown and the peach filling is bubbling at the edges.
08 - Allow the bars to cool completely in the pan on a wire rack. Once fully set, lift the slab out using the parchment overhang, slice into 12 even bars, and serve.