Preheat oven to 350 F. Lightly grease a 9x13 inch cake pan and set aside.

In a light-colored saucepan, melt butter over medium heat. Allow butter to brown in the saucepan, until the butter has a nutty aroma and is about the color of graham crackers. Remove from heat and place in the bowl of a stand mixer with the paddle attachment. Add dark brown sugar, salt, and vanilla and stir until just combined. If mixture is not cool at this point, allow it to sit for a few minutes until cool.

Add eggs one at a time, making sure to scrape the bowl and beat well after each egg. Turn off the mixer and add the flour and baking powder. Mix on low until just combined, then add the white chocolate chips and macadamia nuts. Mix until the chips and nuts are evenly distributed.

Scoop batter into prepared pan and distribute evenly with a spatula. Bake 30-35 minutes until the top is shiny. A toothpick inserted halfway between the side of the pan and the middle of the pan should come out clean. The batter in the middle of the pan will most likely not be set up, and that is completely fine. It will set up as the bars cool down. 

Using a heatproof spatula, press down on the edges of the cookie bars. This will make sure they are nice and even across the top. Allow the bars to completely cool before cutting. You can even let them set overnight, if you can wait that long. Cut cookie bars can be wrapped and stored in an airtight container for up to two days.