Preheat oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

Prepare the base/topping mixture: In a large bowl, whisk together the flour, brown sugar, 1 teaspoon cinnamon, baking powder, and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.

Press two-thirds (about 1.5 cups) of the mixture evenly into the bottom of the prepared baking pan to form the crust. Bake for 15 minutes until lightly set.

Prepare the filling: While the crust bakes, combine the diced apples, granulated sugar, lemon juice, 1 teaspoon cinnamon, and cornstarch in a medium bowl. Toss gently until the apples are evenly coated.

Spoon the apple filling evenly over the partially baked crust. Sprinkle the remaining one-third of the crumb mixture over the apple layer, ensuring an even distribution for the topping.

Bake for 40 to 45 minutes, or until the topping is golden brown and the apple filling appears bubbly around the edges. Allow the bars to cool completely in the pan on a wire rack (at least 2 hours) before lifting out with the parchment paper overhang and slicing.