Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in the cold, cubed butter until the mixture resembles coarse crumbs, then add the egg yolk and ice water. Mix until a dough forms, then press it evenly into a greased 9x13 inch pan. Pre-bake the crust for 15–20 minutes or until lightly golden.

In a medium saucepan over medium heat, combine fresh blackberries and sugar, cooking for about 5–7 minutes until the blackberries release their juices and begin to soften. Remove from the heat and stir in the fresh lemon juice and cornstarch. Return to heat and cook for an additional 1–2 minutes until thickened. Allow this vibrant filling to cool slightly.

In a mixing bowl, whisk together light brown sugar, baking powder, and flour until well combined. Cut in cold butter until the mixture becomes crumbly, then stir in one egg and vanilla extract until mixed. Gently fold in the shelled, unsalted pistachios, ensuring they're evenly distributed throughout the topping mixture.

Pour the blackberry filling over the pre-baked crust, spreading it out evenly. Carefully add the pistachio topping by spooning it over the berry layer and pressing down gently to ensure it adheres. Bake for 30–35 minutes or until the topping is golden brown and the filling is bubbly.

Once baked, remove the pan from the oven and allow the bars to cool in the pan for 1–2 hours. After cooling, slice the Blackberry Pistachio Dream Bars into squares and enjoy!