01 - Preheat oven to 350°F. Line 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Whisk together flour, baking powder, and salt in a medium bowl until evenly blended.
03 - Mix melted butter and brown sugar in large bowl until smooth. Add egg and vanilla extract, whisking until fully incorporated.
04 - Gently fold dry ingredients into wet mixture until just combined. Do not overmix.
05 - Stir in chopped pistachios and white chocolate chips until evenly distributed throughout batter.
06 - Transfer batter to prepared pan, spreading evenly into corners with spatula.
07 - Bake for 22-25 minutes until edges are golden brown and toothpick inserted in center comes out with moist crumbs.
08 - Cool completely in pan on wire rack. Lift out using parchment overhang and cut into squares.