Preheat your oven to 350°F (175°C). Grease or line a baking pan with parchment paper.

In a mixing bowl, whisk together flour, sugar, and salt for the crust.

Cut in the cold butter until the mixture resembles coarse crumbs.

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

Press the dough evenly into the prepared pan.

Bake for 15–20 minutes or until lightly golden.

While the crust bakes, prepare the filling by combining blackberries and sugar in a saucepan over medium heat.

Cook for 5–7 minutes until the berries release their juices.

Stir in lemon juice and cornstarch. Cook for another 1–2 minutes until thickened.

Remove from heat and allow the mixture to cool slightly.

For the topping, whisk together brown sugar, baking powder, and flour.

Cut in the butter until crumbly.

Mix in the egg and vanilla extract.

Fold in the pistachios until evenly distributed.

Remove the crust from the oven and spread the blackberry filling evenly over the surface.

Sprinkle and gently press the pistachio topping over the filling.

Bake the Blackberry Pistachio Dream Bars for 30–35 minutes until golden brown and bubbling.

Cool completely before slicing into bars and serving.