In a medium bowl, combine the sliced ripe peaches, granulated sugar, cornstarch, vanilla extract, and lemon juice. Gently toss the ingredients together until the peaches are evenly coated. Set the mixture aside to allow the flavors to meld while you prepare the crumble.

Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan. In a large mixing bowl, whisk together the all-purpose flour, rolled oats, brown sugar, granulated sugar, ground cinnamon, baking powder, and salt. Pour in the melted unsalted butter, mixing until the mixture resembles coarse crumbs.

Divide the crumble mixture in half, pressing the first half firmly into the bottom of the prepared baking pan to form an even layer. Next, pour the peach filling over the pressed crumble base, spreading it evenly. Crumble the remaining mix over the peach filling, ensuring the fruit is well covered.

Place the pan in the preheated oven and bake for about 30 minutes. You'll know the Cinnamon Peach Crumble Dessert Bars are done when the top is golden brown and the peach filling is bubbling around the edges.

Once baked, remove the pan from the oven and let it cool for at least 10 minutes. This cooling time helps the bars firm up, making slicing easier. Once cooled slightly, cut into squares and serve warm or at room temperature, possibly with a scoop of vanilla ice cream.