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

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, and salt.

Cut in the cold, cubed unsalted butter until the mixture resembles coarse crumbs.

Add the egg yolk and ice water, mixing until a dough forms.

Press this mixture evenly into the prepared pan, and pre-bake for 15-20 minutes, or until it's lightly golden.

In a medium saucepan over medium heat, combine fresh blackberries and granulated sugar, cooking for 5-7 minutes until the berries release their juices.

Stir in the lemon juice and cornstarch; continue cooking for another 1-2 minutes until thickened.

Remove the saucepan from heat and let it cool slightly before pouring over the crust.

In a separate bowl, whisk together light brown sugar, baking powder, and a little more flour.

Cut in additional cold butter until the mixture is crumbly.

Then, mix in a large egg and vanilla extract until fully combined.

Gently fold in the shelled pistachios, ensuring they’re evenly distributed.

Pour the blackberry filling evenly over the pre-baked crust.

Dollop the pistachio topping over the blackberry filling, pressing it down gently.

Bake the assembled bars for 30-35 minutes, or until the topping is golden brown and filling is bubbly.

After baking, allow the bars to cool completely in the pan for at least 1-2 hours.

Once cool, lift the bars out and slice into squares or rectangles.