01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving enough overhang on two sides to lift the bars out easily after baking.
02 - In a large mixing bowl, beat the softened butter and granulated sugar together until pale and fluffy. Add the flour and salt, then mix on low speed until a crumbly dough forms and begins to come together.
03 - Press the dough evenly into the bottom of the prepared pan, creating a flat, uniform layer. 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, granulated sugar, cornstarch, and lemon juice in a medium bowl. Toss gently until the peaches are evenly coated and the cornstarch has 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 fingertips until the mixture forms coarse, pea-sized crumbs.
06 - Spread the peach filling in an even layer over the warm par-baked crust. Scatter the streusel topping evenly across the surface, covering the peaches completely.
07 - Return the pan to the oven and bake for an additional 30 minutes, or until the streusel is deeply golden 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 set, use the parchment overhang to lift the slab out. Slice into 12 even squares and serve at room temperature or chilled.