Preheat the oven to 350°F (175°C) and lightly grease a 9x9-inch baking dish.

In a mixing bowl, combine melted butter, granulated sugar, flour, and salt. Mix until crumbly and press into the bottom of the dish. Bake for about 10 minutes until golden.

In another bowl, whisk together the eggs and brown sugar until fluffy, then fold in blackberries, pistachios, and vanilla.

Pour the filling over the crust and bake for 25-30 minutes or until the edges are golden brown.

Let the bars cool completely at room temperature for 1-2 hours, then cut into squares.

Serve at room temperature or chilled with optional whipped cream or powdered sugar.