Preheat your oven to 375°F. Prepare a 9x13-inch light metal baking pan by lining it with foil or parchment paper, allowing it to extend over the edges. Lightly spray the lining with cooking spray and set it aside.
For the crust, mix together the granulated sugar, baking powder, salt, and all-purpose flour in a large bowl until well combined.
Add the cold, cubed butter, lightly beaten egg, and vanilla extract to the dry ingredients. Use a pastry cutter or two forks to blend the mixture until it resembles coarse crumbs or small pebbles. The mixture should be crumbly.
Press half of the crust mixture evenly into the bottom of the prepared pan, using your hands to create a smooth, even layer. Set this aside for a moment.
In another large bowl, combine the diced peaches, granulated sugar, cornstarch, lemon juice, and cinnamon. Toss everything together with a spoon until the peaches are well coated.
Spread the peach filling evenly over the crust in the pan. Then, sprinkle the remaining crumble topping generously over the peach layer.
Bake in the preheated oven for 38-45 minutes, or until the topping is a light golden brown, bubbly, and the center looks set.
Allow the bars to cool completely at room temperature for 1-2 hours before cutting them into squares. Alternatively, you can refrigerate them to cool.
