Preheat the oven to 350 degrees Fahrenheit. Line a 9x9-inch baking pan with parchment paper and set aside.
In a large bowl, whisk together the flour, baking powder and salt. Add the cold butter and use a pastry cutter to create a crumb-like mixture. Alternatively, the mixture can be combined in a food processor pulsing until pea-sized crumbs form, about 20 seconds.
In a separate large bowl, whisk together the brown sugar, egg and vanilla until combined. Slowly add the flour mixture to the wet mixture and using a rubber spatula (or your hands) combine ingredients into a crumbly mixture. Press 2/3 of the mixture into the bottom of prepared baking pan. Make sure it is compact so it creates a sturdy crust for the peaches.
In a separate bowl, stir together the chopped peaches, sugar, cornstarch and lemon juice. Spoon peach mixture over crust. Top with remaining crumble mixture.
Bake for 40 to 45 minutes or until the crumble top is golden brown and the peach layer is bubbling at the edges. Remove from oven and allow to cool completely at room temperature. Cut bars into 1-1/2 inch squares and serve.
Notes
Storage: Store leftover bars in an airtight container in the fridge for up to 4 days. They will hold their shape better and last longer when stored in the refrigerator.