Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease the parchment paper with non-stick cooking spray or butter.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, combine the melted butter, light brown sugar, and granulated sugar. Mix until well combined and smooth.
Add the egg and beat well. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix the batter.
Gently fold in the white chocolate chips and chopped macadamia nuts until evenly distributed throughout the dough.
Pour the dough into the prepared baking pan, using a spatula to spread it out evenly.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Allow the cookie bars to cool completely in the pan on a wire rack. Once cool, use the parchment paper overhang to lift the bars out of the pan and onto a cutting board.
