Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the long sides to lift the bars out later. Lightly spray the parchment with nonstick spray.

In a large mixing bowl, whisk together the melted, slightly cooled butter, brown sugar, and granulated sugar until smooth and glossy, about 30–45 seconds. Add the eggs and vanilla and whisk again until well combined and slightly thickened.

In a separate medium bowl, whisk together the flour, baking soda, baking powder, and salt until evenly combined.

Add the dry ingredients to the wet ingredients. Using a spatula or wooden spoon, stir gently until just combined and no dry flour remains. The batter will be thick, similar to cookie dough.

Fold in the white chocolate chips or chunks and the chopped macadamia nuts. If desired, reserve a small handful of chips and nuts to sprinkle over the top before baking.

Scoop the thick batter into the prepared pan. Use a spatula or lightly greased hands to press it into an even layer, reaching all the corners. Sprinkle any reserved white chocolate and macadamia nuts over the top and gently press them into the surface.

Bake on the center rack for 22–28 minutes, or until the edges are lightly golden and the center looks set but still slightly soft. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter. For softer, gooier bars, bake closer to 22–24 minutes.

Place the pan on a wire rack and let the bars cool completely in the pan so they set and slice cleanly. To speed this up, you can cool them in the refrigerator for 30–45 minutes.

Use the parchment overhang to lift the slab of bars out of the pan. Transfer to a cutting board and slice into 16–20 squares. If desired, sprinkle lightly with flaky sea salt just before serving.