Step 1: Preheat the Oven and Prepare the Pan

Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal. Grease the parchment with non-stick spray or butter.

Step 2: Make the Crust and Crumble

In a large bowl, whisk together the flour, rolled oats, granulated sugar, brown sugar, baking powder, salt, and ground cinnamon (if using). Add the melted butter and vanilla extract, and stir until the mixture resembles coarse crumbs. The mixture should be crumbly but hold together when pressed.

Step 3: Assemble the Crust

Take about two-thirds of the crumb mixture and press it firmly into the bottom of the prepared baking pan, creating an even layer. Use the bottom of a measuring cup or a flat spatula to press the crust firmly into the pan. This will form the base of your bars.

Step 4: Prepare the Blackberry Filling

In a medium bowl, toss the blackberries with the granulated sugar, cornstarch, lemon juice, and lemon zest (if using). The cornstarch helps thicken the juices from the blackberries as they bake, preventing the bars from becoming soggy.

Step 5: Assemble the Bars

Spread the blackberry mixture evenly over the crust. Be sure to distribute the blackberries evenly, so every bite has a burst of juicy fruit. Sprinkle the remaining crumb mixture over the top of the blackberries, leaving some of the fruit exposed for a rustic look.

Step 6: Bake the Bars

Place the pan in the preheated oven and bake for 35-40 minutes, or until the crumble topping is golden brown and the blackberry filling is bubbly. The top should be crispy and the edges slightly browned.

Step 7: Cool and Slice

Once baked, remove the pan from the oven and let the bars cool completely in the pan. Once cooled, use the parchment paper overhang to lift the bars out of the pan and transfer them to a cutting board. Slice into 12-16 bars, depending on your preferred size.