In a spacious mixing bowl, combine the flour, cornstarch, baking soda, and salt. Whisk until these dry ingredients are fully incorporated and set the bowl aside.

In a separate bowl, whisk together the melted butter, both sugars, the whole egg, egg yolk, and vanilla extract until the mixture is smooth and the sugars have dissolved. Gradually pour this wet mixture into the bowl with the dry ingredients. Using a silicone spatula, mix until the dough forms a uniform consistency. Gently fold in the white chocolate chips and chopped macadamia nuts. If preferred, an electric mixer can be used for this step.

Cover the bowl with plastic wrap and chill the cookie dough in the refrigerator for at least 2 hours, or up to 4 days. If you chill for longer than 2 hours, let the dough sit at room temperature for about 20-30 minutes before rolling and baking, as it will become firm.

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

Scoop and roll the dough into balls, roughly 1 to 1.5 tablespoons each. Arrange them on the prepared baking sheets, leaving about 3 inches of space between each ball. Bake in the preheated oven for 12-13 minutes, or until the edges are lightly golden brown while the centers remain soft.

Take the cookies out of the oven and let them rest on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.

Store the cookies in an airtight container at room temperature; they’ll stay fresh for up to a week.