Preheat the oven to 180 degrees Celsius. Line a 9x13 inch baking pan with parchment paper, leaving extra over the sides for easy removal.

In a large bowl, combine flour, sugar, baking powder, salt, and cinnamon. Mix well so the ingredients are evenly distributed.

Add melted butter and the egg to the dry mixture. Stir using a fork or spatula until a crumbly dough forms. The mixture should resemble coarse crumbs and hold lightly when pressed.

Transfer two thirds of the crumb mixture into the prepared pan. Press it down firmly and evenly to create a solid base layer. Make sure all edges are covered.

In another bowl, mix diced peaches, sugar, lemon juice, and cornstarch. Stir gently until the peaches are evenly coated and the mixture looks slightly thick.

Spread the peach filling evenly over the base layer. Use a spoon to ensure the fruit is evenly distributed.

Sprinkle the remaining crumb mixture evenly over the top of the peaches. Do not press it down. This will create a light and crumbly topping.

Bake in the preheated oven for 40 to 45 minutes, or until the top is golden brown and the filling is bubbling slightly at the edges.

Remove from the oven and allow the bars to cool completely in the pan. This helps the filling set properly.

Once cooled, lift the bars out using the parchment paper. Place on a cutting board and slice into even squares before serving.