1. Preheat your oven to 350°F (175°C) and line an 8×8-inch baking dish with parchment paper.
2. In a medium bowl, mix together the flour, softened butter, powdered sugar, and salt until a crumbly dough forms.
3. Press the dough evenly into the bottom of your prepared baking dish. Bake for 15 minutes, or until lightly golden.
4. While the crust is baking, whisk together the granulated sugar, eggs, and vanilla extract in another bowl until smooth.
5. Fold in the blackberries, chopped pistachios, and lemon zest, ensuring the berries are evenly distributed.
6. Pour the blackberry mixture over the baked crust, spreading it evenly with a spatula.
7. Bake for an additional 20-25 minutes, or until the top is set and slightly golden.
8. Let the bars cool completely in the pan on a wire rack before slicing into squares. Enjoy them as is or with a dollop of whipped cream!

One personal tip: keep an eye on the bars during the second bake to avoid over-browning. The balance between the soft filling and crunchy crust is key!