Whisk the flour, baking soda, cornstarch, and salt together in a large bowl. Set aside.
In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until smooth. Whisk in the egg, egg yolk, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix together with a rubber spatula.
Fold in the white chocolate and macadamia nuts until evenly distributed. Cover the dough and chill in the refrigerator for at LEAST 3 hours. Your patience will be rewarded. I have popped them in the freezer for 30 minutes before, but you'll always get a better cookie the longer you let it chill.
After it's chilled, removed the dough from the refrigerator and allow to slightly soften 10 minutes. While you wait, preheat oven to 325°F.
Line two large baking sheets with parchment paper and set aside.
Using a medium cookie scoop (highly recommended), scoop out balls of cookie dough. Place 8 balls of dough onto each cookie sheet. Do NOT press down, the height of the ball will prevent the cookies from spreading too much. Bake the cookies for 12 minutes. The cookies will look underbaked, but they're not. Allow them to cool on the cookie sheet for 10 minutes before transferring to a wire rack to cool completely.