01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving a 2-inch overhang on the long sides for easy removal.
02 - In a large mixing bowl, cream the softened butter and granulated sugar together until pale and fluffy, about 2 minutes. Add the flour and salt, mixing on low speed until the dough just comes together in crumbly clusters.
03 - Transfer the dough to the prepared pan and press it firmly into an even layer across the bottom using your hands or the back of a measuring cup. Bake for 15 minutes until the edges are lightly golden. Remove and set aside.
04 - In a medium bowl, toss the diced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Let the mixture rest while preparing the streusel.
05 - In a separate bowl, whisk together the flour, brown sugar, cinnamon, and salt. Pour in the melted butter and stir with a fork until the mixture forms moist, uneven crumbs ranging from pea-sized to walnut-sized pieces.
06 - Spread the peach filling in an even layer over the pre-baked crust, distributing the fruit uniformly. Scatter the streusel topping over the peaches, covering the surface as completely as possible.
07 - Bake for 25 minutes, or until the streusel is deeply golden and the peach filling is bubbling at the edges. The center should look set.
08 - Allow the bars to cool completely in the pan on a wire rack, at least 1 hour. Use the parchment overhang to lift the slab out onto a cutting board, then slice into 12 even squares.