Preheat the oven to 350ºF. Spray a 9x13 baking pan with cooking spray. Line the pan with parchment paper so it hangs a bit over the sides, and then spray the parchment paper with cooking spray.
In a small saucepan, melt butter over medium heat. The butter will begin to crackle and foam a bit. Allow it to cook, swirling the pan occasionally, until the crackling subsides and the butter begins to turn a nutty brown. Remove from the heat and transfer the butter and all the browned bits to a large bowl. Allow the butter to cool slightly.
Once the butter has cooled, whisk in the brown sugar until incorporated. Add the eggs and vanilla and continue to whisk until smooth. Fold in the flour and baking powder until the dry ingredients are no longer visible. Fold in the pistachios and white chocolate chips.
Transfer the batter to the prepared pan and smooth out the top. Bake 22-25 minutes or until the edges are just beginning to brown and a toothpick inserted in the center comes out clean. You don’t want to over bake them or they will be dry. Allow the bars to cool before using the parchment to lift them out of the pan. Cut into squares and serve.