Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in cold, cubed unsalted butter until the mixture resembles coarse crumbs. Add the egg yolk and ice water, mixing until a dough forms. Press into a greased 9x13 inch baking pan and bake for 15-20 minutes until lightly golden.

For the filling, combine fresh blackberries and sugar in a saucepan over medium heat. Cook for about 5-7 minutes until the blackberries release their juices. Stir in lemon juice and cornstarch, cooking for an additional 1-2 minutes until thickened. Allow to cool slightly before spreading over the crust.

In a separate bowl, whisk together light brown sugar, baking powder, and flour. Cut in cold butter until crumbly. Add the egg and vanilla extract, stirring until just combined. Fold in shelled pistachios.

Spread the cooled blackberry filling evenly over the baked crust. Sprinkle the pistachio topping generously over the filling, pressing gently. Bake for 30-35 minutes until golden brown and bubbling.

Let the pan cool completely on a wire rack for 1-2 hours. Once cooled, lift the bars out and slice into squares or rectangles for serving.