Preheat your oven to 350°F (175°C) and prepare a greased 9x13 inch baking pan.

In a large mixing bowl, combine the all-purpose flour, granulated sugar, and salt. Cut in the cold, cubed unsalted butter until it resembles coarse crumbs.

Add the egg yolk and 2-3 tablespoons of ice water to the mixture, stirring gently until the dough comes together. Press it evenly into the prepared baking pan. Bake for 15-20 minutes until golden brown.

While the crust bakes, combine the fresh blackberries and granulated sugar in a medium saucepan over medium heat. Cook for 5-7 minutes until juicy and bubbling.

Stir in the lemon juice and 1 tablespoon of cornstarch, cooking for an additional 1-2 minutes until the mixture thickens.

In a separate bowl, whisk together the light brown sugar, baking powder, and remaining flour. Cut in the cold butter, then mix in the egg and vanilla extract before folding in the pistachios.

Pour the blackberry filling evenly over the baked crust, then sprinkle the pistachio topping over the blackberry layer.

Bake for 30-35 minutes, until the topping is golden brown and bubbling. Let cool completely for 1-2 hours before slicing.

Slice the bars into squares or rectangles and serve as is, or with powdered sugar or a scoop of ice cream.