Preheat your oven to 350°F (175°C). In a mixing bowl, cream together 1 cup of unsalted butter, 1 cup of all-purpose flour, and 1/2 cup of granulated sugar until smooth. Press this mixture into the bottom of a 9x9-inch baking pan. Bake for 15–20 minutes or until golden.

While the crust is baking, add 2 cups of blackberries to a medium saucepan with 1/2 cup of granulated sugar, 2 tablespoons of lemon juice, and 2 tablespoons of cornstarch. Cook for 5–7 minutes until thickened.

In a separate bowl, combine 1 cup of chopped pistachios, 1/2 cup of rolled oats, 1/4 cup of all-purpose flour, and 1/4 cup of brown sugar. Mix until crumbly, then sprinkle over the cooled blackberry filling.

Bake the assembled bars for an additional 25–30 minutes, until the topping is golden brown and fragrant.

Let the bars cool completely in the pan on a wire rack before slicing into squares.

Serve the bars with a dollop of whipped cream or scoop of vanilla ice cream, if desired.