Melt the butter in a small saucepan over medium heat until completely melted. Turn off the heat and let the butter cool to room temperature, 1 - 2 hours. Transferring the melted butter to the stand mixer bowl should help it cool faster.

Roast the macadamia nuts in a skillet over medium heat for 3 - 5 minutes until toasty and fragrant. Let cool and roughly chop into smaller pieces.

Add brown sugar and granulated sugar to the cooled butter and beat with the paddle attachment for 2 minutes until fluffy and lighter in color.

Add the egg, egg yolk, vanilla extract, and continue beating for another 2 minutes until fluffy.

In another mixing bowl, sift together the flour, baking soda, and baking powder. Stir in the kosher salt and give it a good mix to distribute. Add the flour mixture to the wet ingredient mixture in the stand mixer bowl, stir until incorporated. Stop the mixer and scrape down the sides and bottom of the bowl as necessary.

Portion the cookie dough with a large cookie scoop and place it on a parchment lined baking sheet. Refrigerate for at least 1 hour, and up to 2 - 3 hours before baking.

Optional: If you want the cookie to be rounder, roll the portioned dough into balls. If the dough is too soft right after mixing for rolling, refrigerate it for 30 minutes, then roll into balls.

Preheat the oven to 350°F.

Place 6 dough balls on a baking sheet, about 2 inches apart and bake 1 batch at a time for 15 minutes, until the outer edges of the cookies are golden brown and the center is still slightly gooey.

Allow the cookies to cool on the baking sheet for 10 minutes before removing them to a wire rack to cool completely.