01 - Preheat the oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the bars out after baking.
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 in a crumbly mass.
03 - Press the dough evenly into the bottom of the prepared pan. Bake for 15 minutes, or 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 the fruit is thoroughly 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, uneven crumbs.
06 - Spread the peach filling in an even layer over the par-baked crust. Scatter the cinnamon streusel evenly across the top, covering the peaches as uniformly as possible.
07 - Bake for 25 minutes, or until the streusel topping is golden brown and the peach filling is bubbling at the edges.
08 - Allow the bars to cool completely in the pan. Use the parchment overhang to lift the entire slab out, then cut into 12 even squares.