Preheat Oven:

Preheat to 350°F (175°C).

Line an 8x8-inch or 9x9-inch baking pan with parchment paper.

Make the Crust/Crumble Mixture:

In a large bowl, cream butter and sugar together until light and fluffy.

Mix in vanilla extract, then add flour and salt.

Stir until crumbly, then fold in chopped pistachios.

Press the Crust:

Press about 2/3 of the dough firmly into the bottom of the prepared pan to form an even crust. Reserve the rest for the topping.

Prepare the Blackberry Filling:

In a small bowl, gently toss the blackberries with sugar, lemon juice, cornstarch, and vanilla.

Spread the mixture evenly over the crust.

Add the Crumble Topping:

Crumble the remaining dough over the blackberry layer. Sprinkle extra chopped pistachios on top if desired.

Bake:

Bake for 35–40 minutes, or until the top is golden and the filling is bubbly.

Let cool completely in the pan before slicing into bars.