In a medium bowl whisk flour, baking soda, salt and cornstarch together. Set aside.
In a large bowl mix butter, brown sugar and granulated sugar together until the butter has lightened. Add in egg and egg yolk and mix until combined. Then stir in the vanilla extract. Add in the dry ingredients and mix until just combined. Stir in the chocolate chips and macadamia nuts.
Cover and chill the dough in the refrigerator for at least 2 hours or overnight. If chilling for more then 2 hours, allow the dough to sit at room temperature for 20 minutes to soften it a bit.
Preheat oven to 350°F and line 2 baking sheets with silicone mats. Roll cookie dough into 1 inch sized balls and arrange on the cookie sheets, 3 inches apart. Flatten the cookie balls slightly and bake for 10-12 minutes or until cookies are just lightly browned on the sides.
Remove cookies from the oven and let them sit for 5 minutes before transferring them to a wire rack to cool completely. Store cookies in an airtight container at room temperature for 1 week.
Chill the cookie dough - I know! You want to bake these NOW. These would spread too much if you baked them right away though. The dough needs time to rest and firm up a bit in the fridge.
Bake one tray at a time - It's better to allow the heat to concentrate on one tray at a time to bake the cookies perfectly.
Try using salted or unsalted macadamia nuts - Either would work for this cookie.