01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing overhang for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, and salt until evenly blended.
03 - In a large bowl, combine melted butter and brown sugar, mixing until smooth. Whisk in egg and vanilla extract 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 - Spread batter evenly into prepared baking pan, smoothing top with 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.