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

In a medium saucepan over medium heat, add blackberries and sugar. Stir until they release their juices (about 5-7 minutes). Remove from heat, add lemon juice and cornstarch, and cook for an additional 1-2 minutes until thickened. Allow to cool slightly before spreading over the pre-baked crust.

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

Spread the blackberry filling over the crust. Sprinkle the pistachio topping evenly over the filling, pressing it down. Bake for 30-35 minutes until golden and bubbly.

Remove from oven and allow to cool completely in the pan for about 1-2 hours before slicing into squares.