In a small bowl, add the dry ingredients- flour, cornstarch, baking soda, and salt. Mix with a fork or whisk to combine and set aside.
In a separate large bowl, add the melted butter, brown sugar, granulated sugar, egg, egg yolk, and vanilla extract. Mix with a nonstick spatula or wooden spoon until combined. Add dry ingredients to the wet mixture and mix again to form thick dough. combined. Fold in the chopped white (or chocolate chips, macadamia nuts, and guava slices.
Cover the dough with cling wrap and place in the refrigerator for at least 2 hours- this is crucial! The cookies will be very thin if you skip this step.
Remove dough from the refrigerator and preheat oven to 350F. Let the dough sit out while the oven preheats since it will need a little time to soften before you scoop. Prepare a baking sheet with parchment paper.
Scoop the cookie dough into balls, 2 tablespoons of dough per cookie and place on the cookie sheet leaving at least 3 inches between the cookies. .Bake for 12-13 minutes, half way through the baking, pull the tray out of the oven and place on any additional guava slices. When edges are slightly browned, but the center looks slightly underdone and soft. Remove from oven and now add any reserved white chocolate and/or macadamia nuts, by gently pressing them into the tops of the baked cookies. Look for any "bare" spots that would look pretty with some extra mix-ins! Remove from the oven and let cookies to cool. Store cookies in an airtight container at room temperature for up to 1 week.