01 - Preheat the oven to 350°F. Line a 9×13-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 pale and fluffy. Add the flour and salt, stirring until the mixture resembles coarse crumbs and just begins to hold together when pressed.
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 from the oven and set aside.
04 - In a medium bowl, toss the diced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until evenly coated. Let the mixture rest while preparing the streusel topping.
05 - In a separate bowl, whisk together the flour, brown sugar, cinnamon, and salt. Pour in the melted butter and toss with a fork until the mixture forms irregular, crumbly clusters.
06 - Spread the peach filling in an even layer over the par-baked crust. Scatter the streusel topping across the surface, covering the peaches completely.
07 - Return the pan to the oven and bake for 25 minutes, until the streusel is deeply golden 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, use the parchment overhang to lift the slab out and cut into 12 squares.