Using a stand or hand mixer, beat butter and sugar in a bowl until creamy, about 2 minutes. Beat in egg and vanilla until combined.
In a separate bowl, stir together flour, salt, baking powder, baking soda, and corn starch. Gradually add flour mixture to butter mixture, beating on low speed until just combined.
Fold in macadamia nuts until just combined (do not overbeat).
Gently fold in 1 cup white chocolate chips. then cover and refrigerate dough for 1 hour.
Scoop heaping tablespoon sized balls of dough then place on a parchment-lined rimmed baking sheet, spacing about 2 inches apart. Press in some more white chocolate chips and macadamia nuts and sprinkle with some flaked salt if desired.
Bake at 350° until lightly browned, 10 to 12 minutes.
Let cool on pan 10 minutes; transfer cookies to a wire rack, and let cool completely.
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
If you don’t want to use a rolling pin to flatten the chocolate after mounding it after each round of baking, you can use a spatula to spread it out. Honestly, using a rolling pin is much easier, because you don’t have to fight with sliding parchment paper. But you do you!
Transfer frozen dough balls to a freezer-safe plastic bag, and stash in the freezer for up to 3 months.