01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large bowl, beat softened butter and granulated sugar together until pale and fluffy. Add flour and salt, mixing until the dough just comes together in coarse crumbs. Press firmly and evenly into the bottom of the prepared pan.
03 - Bake crust for 15 minutes until lightly golden around the edges. Remove from oven and set aside to cool slightly.
04 - In a medium bowl, toss diced peaches with sugar, cornstarch, lemon juice, and vanilla extract until evenly coated. Allow to sit and macerate while preparing the streusel.
05 - In a separate bowl, whisk together flour, brown sugar, cinnamon, and salt. Pour in melted butter and stir with a fork until the mixture forms irregular, 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 fruit completely.
07 - Bake for 25 minutes, or until the streusel topping is deep golden and the peach filling is bubbling at the edges.
08 - Let bars 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 even squares.