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