Preheat the oven to 375 degrees F. Spray a 9×13-inch baking pan with cooking spray.
Peel the peaches and cut into ¾-inch pieces. In a large bowl, combine the peaches, lemon juice, sugar and cornstarch. Set aside.
For the topping: In a separate medium bowl, combine granulated and brown sugars. Stir in flour, baking powder, salt, and ground cinnamon.
Cut in cold butter pieces with a pastry cutter or a fork until the mixture is crumbly. Stir a lightly beaten egg and chopped hazelnuts into the flour mixture.
Press half of the flour mixture into the bottom of the prepared pan. Spread the peach mixture over the crust. Sprinkle the rest of the flour mixture over the peaches.
Bake in the preheated oven for 35-40 minutes, or until the top is light brown. Left the bars cool on a rack and then place in the fridge to chill. This will make the bars easier to cut and remove from the pan. Cut into 32 squares and serve.