In the bowl of a stand mixer, cream together the butter, brown sugar and white sugar with the paddle attachment for about 2 minutes, or until blended. Add in the egg and vanilla and beat to combine. Add in the flour, cornstarch, baking soda and salt until a soft dough forms, then stir in the macadamia nuts and white chocolate chips.
Chill the dough for AT LEAST 1 hour until firm. I like chilling mine for a minimum of 2 hours for best results; you could even chill overnight. Chilling is MANDATORY as it prevents the cookie dough from spreading and helps resolidify the butter for chewy, soft cookies.
Preheat oven to 350 degrees F. Line two baking sheets with silicone liners or parchment paper that's been misted with cooking spray. Drop heaping, rounded Tablespoonfuls of dough onto each cookie sheet, leaving about 2" apart. Bake for approx. 8-10 minutes, rotating pans halfway through baking time to ensure even cooking. Cookies may appear underdone, but they'll continue to cook as they set up. Do NOT overbake.
Allow the cookies to cool completely on the baking sheets before serving. You may top each cookie with additional white chocolate chips for a pretty appearance after removing from the oven, if you'd like.