01 - Preheat oven to 350°F. Line a 9x13-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 granulated sugar together until pale and fluffy. Add flour and salt, mixing on low speed until the dough just comes together in coarse crumbs.
03 - Transfer the dough to the prepared pan and press it firmly into an even layer across the bottom. Bake for 15 minutes until the edges are lightly golden. Remove and set aside.
04 - In a medium bowl, toss diced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until evenly coated. Let the mixture rest while preparing the streusel.
05 - In a separate bowl, whisk together flour, brown sugar, cinnamon, and salt. Pour in melted butter and toss with a fork until the mixture forms moist, crumbly clumps.
06 - Spread the peach filling in an even layer over the par-baked crust. Scatter the cinnamon streusel across the top, covering the peaches completely.
07 - Bake for 25 minutes, or until the streusel is deeply golden and the peach filling bubbles around the edges.
08 - Cool the bars completely in the pan on a wire rack. Use the parchment overhang to lift the slab out, then cut into 12 even squares.