Whisk the flour, cornstarch, baking soda, and salt together in a large bowl. Set aside.

Whisk the melted butter, brown sugar, granulated sugar, egg, egg yolk, and vanilla extract together until combined. Pour into dry ingredients and mix everything together with a silicone spatula until completely combined. Fold in the white chocolate chips and macadamia nuts. (You can use a mixer for this step if needed.)

Cover and chill the dough in the refrigerator for at least 2 hours and up to 4 days. If chilling for longer than 2 hours, allow to sit at room temperature for at least 20-30 minutes before rolling and baking because the dough will be quite hard.

Preheat oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.

Roll cookie dough into balls, about 1-1.5 Tablespoons of dough per cookie (I like to use this medium cookie scoop), and arrange 3 inches apart on the baking sheets. Bake for 12-13 minutes or until lightly browned on the sides. The centers will look soft.

Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.