Preheat the oven to 350°F / 175°C. Line a baking sheet with parchment paper or with a silicone mat.
In a medium sized microwave safe bowl, melt the butter in 20 second intervals, stirring after each interval.
Add the light brown sugar to the butter and whisk until combined.
Add in the egg yolk and vanilla, whisking until thoroughly combined.
Add in the flour, baking soda, baking powder and salt. Lightly fold together using a rubber spatula until the flour is almost completely combined. Add in the white chocolate and macadamia nuts, then fold just until no flour streaks remain.
Use a 3 tbsp. cookie scoop to dish out the cookies and evenly space them out on the baking sheet. Top with extra white chocolate and nuts if desired. Bake for 10 minutes, or until the edges are golden and the middles of the cookies are almost completely set.