Preheat your oven to 350°F (175°C).

Combine all-purpose flour, granulated sugar, and salt in a mixing bowl. Cut in the cold, cubed butter until the mixture resembles coarse crumbs. Add the egg yolk and ice water, mixing gently until a dough forms. Press this dough evenly into a greased 9x13 inch pan and pre-bake for 15-20 minutes, until lightly golden.

In a medium saucepan over medium heat, combine fresh blackberries and granulated sugar, stirring until the berries release their juices (about 5-7 minutes). Once bubbling, add lemon juice and cornstarch, stirring well. Cook for an additional 1-2 minutes until thickened. Remove from heat and let cool slightly.

In a separate bowl, whisk together light brown sugar, baking powder, and all-purpose flour. Cut in the cold butter until crumbly. Add the egg and vanilla extract, mixing until just combined; fold in the chopped pistachios.

Pour the cooled blackberry filling over the pre-baked crust. Add the pistachio mixture on top, pressing down gently. Bake for 30-35 minutes until golden brown and filling is bubbling.

Allow the bars to cool completely in the pan for at least 1-2 hours before slicing.