Preheat the oven to 350°F (175°C) and line a 9×9-inch baking pan with parchment paper or lightly grease it.
In a large mixing bowl, whisk together the melted butter and brown sugar until smooth and well combined.
Beat in the eggs, one at a time, followed by the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the white chocolate chips and chopped pistachios, ensuring even distribution.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the blondies cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.
Once cooled, cut into squares and enjoy!