Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, and salt. Cut the cold, cubed butter into the flour mixture until it resembles coarse crumbs. Add the egg yolk and ice water, mixing gently until a dough forms. Press the crust evenly into the prepared pan and pre-bake for 15-20 minutes, until it's lightly golden.

In a medium saucepan over medium heat, combine fresh blackberries and some sugar. Cook for 5-7 minutes until the blackberries release their juices and soften. Stir in the fresh lemon juice and cornstarch, cooking for an additional 1-2 minutes until the mixture thickens and becomes bubbly. Remove the saucepan from heat and let the filling cool slightly as you prepare the topping.

In a mixing bowl, whisk together the brown sugar, baking powder, and flour for the topping. Cut in the cold butter until the mixture becomes crumbly. Beat in a large egg and pour in the vanilla extract, mixing well. Finally, fold in the shelled pistachios. Set the pistachio topping aside until you’re ready to assemble.

Once your crust has pre-baked, pour the cooled blackberry filling evenly over the warm crust. Carefully sprinkle the pistachio topping over the filling, pressing it down with your hands for an even layer. Bake in the preheated oven for 30-35 minutes, until the topping is golden brown and the filling is bubbling.

Remove the baking pan from the oven and let the Blackberry Pistachio Dream Bars cool completely, ideally for 1-2 hours, in the pan on a wire rack. Once cooled, slice into squares and serve, enjoying the beautiful combination of textures and flavors!