01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, and salt until evenly blended.
03 - In a large bowl, mix melted butter and brown sugar until smooth. Add egg and vanilla extract, whisking until fully incorporated.
04 - Gently fold dry ingredients into wet mixture until just combined. Avoid overmixing to prevent dense texture.
05 - Stir in chopped pistachios and white chocolate chips until evenly distributed throughout batter.
06 - Transfer batter to prepared pan and spread evenly into corners using spatula.
07 - Bake for 22-25 minutes until edges are golden 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.