Preheat the oven to 350°F (175°C) and prepare an 8x8-inch baking dish.

Make the crust by combining melted butter, granulated sugar, and flour until crumbly. Add salt and baking powder, then press evenly into the baking dish.

Prepare the filling by whisking eggs and brown sugar until light and fluffy. Fold in blackberries, vanilla, and chopped pistachios.

Pour the filling over the crust and spread evenly. Bake for 25-30 minutes until set.

Cool completely on a wire rack before cutting into squares.

Serve chilled or at room temperature, optionally garnished with powdered sugar or ice cream.