Preheat your oven to 350°F (175°C) and grab a greased 9x13 inch baking pan. In a mixing bowl, whisk together all-purpose flour, granulated sugar, and salt until combined.

Cut in the cold, cubed butter until the mixture resembles coarse crumbs. Mix in the egg yolk and ice water until a dough forms, then press it evenly into the pan. Pre-bake the crust for 15-20 minutes until lightly golden.

While the crust bakes, prepare the filling. Combine fresh blackberries and sugar in a saucepan over medium heat. Cook for about 5-7 minutes until the berries start to burst.

Stir in the lemon juice and cornstarch, cooking until thickened. Remove from heat and let cool slightly.

In a separate bowl, whisk together brown sugar, baking powder, and flour until mixed. Cut in butter until crumbly, then add the egg and vanilla extract, mixing until just combined.

Gently fold in the shelled pistachios. Once the crust is pre-baked, pour the blackberry filling evenly over it, then sprinkle the pistachio topping.

Press down gently on the topping and bake for 30-35 minutes until golden and bubbly.

Let the bars cool for 1-2 hours in the pan before lifting them out and slicing into squares.