Preheat the oven to 350°F. Grease or line with parchment paper an 8 X 8 inch square tin. Set aside.
Add the flour and baking powder to a mixing bowl and mix until well combined. Set aside.
Add the brown sugar and melted butter to a large mixing bowl. Mix for at least 1-2 minutes, until well combined. Add the eggs and vanilla extract and mix for 2-3 minutes until all the ingredients are well combined.
Pour the flour mixture into the bowl with the wet ingredients. Stir until just combined. Fold through the white chocolate chips and chopped macadamia nuts.
Spoon the dough into the prepared baking pan and smooth the surface gently with the back of a spoon. Bake for 20 minutes, or until beginning to turn golden.
Allow to cool in pan completely before removing and slicing.