01 - Preheat the oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving a 2-inch overhang on the long sides for easy removal.
02 - In a large mixing bowl, beat the softened butter and granulated sugar together until pale and fluffy. Add the flour and salt, mixing on low speed until the dough just comes together in coarse crumbs. Transfer the mixture to the prepared pan and press it firmly and evenly across the bottom.
03 - Bake the crust for 15 minutes, or until the edges are lightly golden. Remove from the oven and set aside to cool slightly while preparing the filling.
04 - In a medium bowl, toss the diced peaches with granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Allow the mixture to rest and macerate for 5 minutes.
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 irregular, pebble-sized crumbs.
06 - Spread the peach filling in an even layer over the par-baked crust. Scatter the cinnamon streusel across the top, covering the fruit 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. Once fully set, use the parchment overhang to lift the slab out of the pan and cut into 12 even squares.