Preheat the oven to 350°F and line an 8x8-inch pan with parchment paper.
In a mixing bowl, stir the melted butter and brown sugar until smooth.
Add the egg and vanilla extract, then mix until fully combined.
Whisk in the flour, baking powder, and salt until no dry streaks remain.
Fold the chopped macadamia nuts and dark chocolate chips into the batter.
Spread the batter evenly in the prepared pan.
Bake for 20 to 22 minutes, until the edges turn golden and a toothpick comes out with a few moist crumbs.
Cool completely in the pan before slicing into bars.