Preheat oven to 350°F.  In the bowl of a stand mixer, use the paddle attachment to cream together the softened butter and sugar on medium-high speed until light and fluffy, about 3 minutes.  Add egg and vanilla, and mix until combined, about 1 minute.

Add in the flour, cornstarch, baking soda and salt, and beat on medium-low speed until combined. Fold in (stir in) the chopped macadamia nuts and chocolate chips by hand until just combined.

Use a large cookie scoop (equivalent to 3 tablespoons) or a spoon to shape the dough into balls, and place them on a parchment-covered baking sheet. Sprinkle each with a bit of the flaky sea salt. Chill in the refrigerator for at least 10-20 minutes, or until the dough is chilled completely through.  (This will help prevent the cookies from spreading out too flat when they bake.)

Bake for 10-12 minutes until the edges are just set. The centers may look slightly undercooked. Cool cookies on the baking sheet for about 5 minutes, then transfer to a wire rack to finish cooling. Serve immediately or store in a sealed container for up to 1 week. You can also freeze the cookies for up to 3-4 months.