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 bowl, cream the softened butter and granulated sugar together until light and fluffy. Add the flour and salt, mixing until the dough just comes together. Press the mixture evenly and firmly into the bottom of the prepared pan.
03 - Bake the crust for 15 minutes, until lightly golden around the edges. Remove from the oven and set aside to cool slightly.
04 - In a medium bowl, toss the diced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until the peaches are evenly coated. Set aside to macerate 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 coarse, crumbly clumps.
06 - Spread the peach filling in an even layer over the pre-baked crust. Scatter the cinnamon streusel evenly across the top, covering the peaches completely.
07 - Bake for 25 minutes, or until the streusel topping is deep golden and the peach filling is bubbling around the edges.
08 - Allow the bars to cool completely in the pan on a wire rack. Use the parchment overhang to lift the slab out, then cut into 12 even squares.