Whisk together flour, cornstarch, baking soda, and salt in a large bowl.
In another bowl, whisk melted butter, brown sugar, granulated sugar, egg, egg yolk, and vanilla extract until combined.
Combine wet and dry ingredients, then fold in white chocolate chips and macadamia nuts.
Cover and chill dough in the refrigerator for at least 2 hours (up to 4 days).
Preheat oven to 350°F (177°C) and line baking sheets with parchment paper.
Roll dough into 1-1.5 tablespoon balls and place 3 inches apart on baking sheets.
Bake for 12-13 minutes, cool on baking sheet for 5 minutes, then transfer to a wire rack.