01 - Preheat 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, combine the diced peaches, granulated sugar, cornstarch, lemon juice, and vanilla extract. Stir gently until the peaches are evenly coated and set aside to macerate.
05 - In a separate bowl, whisk together the flour, packed light brown sugar, ground cinnamon, and salt. Pour in the melted butter and toss with a fork until the mixture forms coarse, crumbly pieces.
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 completely.
07 - Bake for 25 minutes, or until the streusel topping is deep golden and the peach filling is bubbling at 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.