In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large bowl using a handheld electric mixer or the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium speed until creamy, about 2 minutes. Beat in the egg and vanilla until combined. Stop and scrape down the bowl.

While mixing on low speed, gradually add the flour mixture to the butter mixture, beating until the flour is fully incorporated. Scrape down the bowl again. Using a silicone spatula, gently fold in 1 cup (180g) of the white chocolate chips and the macadamia nuts.

Scoop heaping tablespoon-sized balls of dough and place them on a small baking sheet. Press the remaining ¼ cup (45g) of white chocolate chips and a few extra chopped macadamia nuts, if desired, into the tops. Cover and refrigerate the dough balls for 1 hour.

After 40 minutes of chilling, preheat the oven to 350°F. Line two large baking sheets with parchment paper.

Place the cookies on the prepared baking sheets, spacing them about 2 inches apart. Bake one sheet at a time for 10 to 12 minutes, or until the bottom edges are lightly browned. Let the cookies cool on the pan for 10 minutes before transferring to a wire rack to cool completely.