01 - Preheat the oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the flour and salt, mixing until a crumbly dough forms.
03 - Press the dough evenly into the bottom of the prepared pan. Bake for 15 minutes until lightly golden. Remove from the oven and set aside.
04 - While the crust bakes, combine the diced peaches, granulated sugar, cornstarch, and lemon juice in a bowl. Toss gently until the peaches are evenly coated.
05 - In a separate bowl, combine the flour, brown sugar, cinnamon, and salt. Cut in the cold cubed butter using a pastry cutter or your fingers until large, coarse crumbs form.
06 - Spread the peach filling evenly over the warm baked crust. Sprinkle the cinnamon streusel topping uniformly over the peach layer.
07 - Return the pan to the oven and bake for an additional 30 minutes, or until the streusel is golden brown and the peach filling is bubbling at the edges.
08 - Allow the bars to cool completely in the pan. Lift the slab out using the parchment overhang, slice into 12 even bars, and serve.