Heat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, cornstarch, baking soda, and salt in a bowl; set aside.
Cream softened butter, brown sugar, and granulated sugar until light and fluffy, 2–3 minutes.
Beat in vanilla, then the beaten egg and yolk until smooth and glossy.
Mix dry ingredients into the butter mixture on low just until a soft dough forms.
Fold in white chocolate chips and chopped macadamias until evenly distributed.
Scoop 1.5-tbsp mounds onto prepared sheets, spacing about 2 inches apart.
Bake 9–12 minutes, until edges are set and centers look slightly soft.
Cool on the sheet 5 minutes, then transfer to a rack to finish cooling.