• Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, leaving a bit of overhang on the sides to make lifting the bars out easier later.

Make the Crust and Crumble:

• In a large bowl, mix the flour, white sugar, and a small pinch of salt.
• Add the cold, cubed butter and use a pastry cutter or your hands to work the butter into the dry ingredients until the mixture forms coarse crumbs. If you're adding oats, mix them in now.
• Press half of the mixture firmly into the bottom of your prepared baking dish to form a solid crust.

Prepare the Blackberry Filling:

• In another bowl, toss the blackberries with granulated sugar, lemon zest, lemon juice, cornstarch, and vanilla extract. Gently toss the berries until they're thoroughly and evenly coated.

Assemble the Pie Bars:

• Spread the blackberry filling evenly over the crust. Scatter the rest of the crumble mixture evenly over the berries.
• Bake in the preheated oven for 35-40 minutes until the top turns golden brown, and the blackberry filling starts to bubble.
• Please remove it from the oven and let the bars cool thoroughly in the dish before slicing.
• Once the bars have cooled, use the parchment paper overhang to lift them out of the dish easily. Slice into squares and serve.