Preheat Oven: Preheat your oven to 350°F. Line baking sheets with parchment paper or silicone baking mats.
Cream Butter and Sugars: In a large mixing bowl, use a hand mixer or stand mixer to beat the butter, granulated sugar, and light brown sugar together on medium speed until light and fluffy, about 2-3 minutes.
1 cup unsalted butter, 1 cup granulated sugar, ⅓ cup light brown sugar
Add Eggs and Vanilla: Add the egg, egg yolk, and vanilla extract. Beat until fully incorporated.
1 large egg, 1 large egg yolk, 1 teaspoon vanilla extract
Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, and cornstarch. Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.
2 ¼ cups all-purpose flour, 1/2 teaspoon baking soda, 2 teaspoons cornstarch
Fold in Mix-Ins: Gently fold in the macadamia nuts, white chocolate chips, and semi-sweet chocolate chips until evenly distributed.
3/4 cup chopped macadamia nuts, 3/4 cup white chocolate chips, 1/2 cup semi-sweet chocolate chips
Shape the Cookies: Scoop about 4 tablespoons of dough for each cookie and roll into balls. Place the dough balls on the prepared baking sheets, spacing them about 4 inches apart.
Bake: Bake for 10-12 minutes, or until the edges are set and the centers look slightly underbaked. Do not overbake for a soft and chewy texture.
Cool: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.