Preheat your oven to 350°F (175°C). Grease or line a 9x9-inch baking dish with parchment paper.
Make the Crust and Topping:
In a mixing bowl, combine flour, rolled oats, brown sugar, baking powder, and salt. Mix well.
Pour in the melted butter and vanilla extract, then mix until crumbly.
Prepare the Peach Filling:
In another bowl, combine the sliced peaches, granulated sugar, cornstarch, lemon juice, and cinnamon (if using). Toss until the peaches are evenly coated.
Assemble the Bars:
Press half of the crumb mixture into the bottom of the prepared baking dish to form a crust.
Spread the peach mixture evenly over the crust.
Sprinkle the remaining crumb mixture on top of the peaches.
Bake:
Bake in the preheated oven for 30-35 minutes or until the topping is golden brown and the filling is bubbly.
Cool and Serve:
Allow to cool for at least 10 minutes before cutting into squares. Serve warm or at room temperature.