Preheat your oven to 375°F (190°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides to act as a “sling.”
In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon. Ensuring these dry ingredients are fully combined is a professional-grade step that prevents any leavening clumps.
Add the cold, cubed butter to the flour mixture. Use a pastry cutter to blend until the mixture looks like coarse sand. Stir in the beaten egg until the dough begins to clump. Keeping the butter cold is a top-notch secret; it prevents the dough from becoming greasy and ensures the shortbread base remains flaky.
Firmly press about two-thirds of the dough into the bottom of your prepared pan. Use a flat-bottomed cup to pack it down into an even, solid layer. Place the remaining third of the dough in the fridge to keep it chilled and firm.
Gently toss the blackberries with sugar, cornstarch, lemon juice, and vanilla. Spread the berries over the crust in an even layer, ensuring they reach all the way to the corners of the pan.
Remove the remaining dough from the fridge and crumble it over the berries with your fingers. Bake for 45 to 50 minutes until the top is golden and the fruit juices are bubbling. You must let the bars cool completely in the pan for at least two hours; this allows the jam to set so the bars don’t fall apart when sliced.
