01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing overhang for easy removal.
02 - Whisk together flour, baking powder, and salt in a medium bowl until evenly distributed.
03 - In a large bowl, combine melted butter and brown sugar, stirring until smooth. Add egg and vanilla extract, whisking until fully incorporated.
04 - Fold dry ingredients into wet mixture until just combined. Avoid overmixing to maintain tender texture.
05 - Stir in chopped pistachios and white chocolate chips until evenly distributed throughout batter.
06 - Spread batter evenly in prepared pan. Bake for 22-25 minutes until edges are golden and toothpick inserted in center comes out with moist crumbs.
07 - Cool completely in pan on wire rack. Lift out using parchment overhang and cut into squares.