Preheat your oven to 375°F and line a 9x13-inch pan with parchment paper. In a large bowl, whisk together flour, sugar, and salt. Cut in cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces.

Whisk egg yolk with cold water, then drizzle over the flour mixture. Gently stir until the dough just comes together - it should look slightly shaggy. Press two-thirds of this mixture firmly into your prepared pan, creating an even layer that goes slightly up the sides.

Gently toss blackberries with sugar, cornstarch, lemon juice, vanilla, and cinnamon in a medium bowl. Let this mixture sit for 5 minutes while the berries release their juices.

Spread the blackberry mixture evenly over the pressed crust. Crumble the remaining dough mixture on top - don't worry about perfect coverage, as rustic gaps let the beautiful purple filling peek through. Bake for 40-45 minutes until the top is golden brown and the filling bubbles around the edges.

Let the bars cool completely in the pan before cutting - this takes about 2 hours but ensures clean slices. The filling needs time to set properly, transforming from bubbly liquid to perfect jammy consistency.