Place an oven rack on the center shelf. Preheat the oven to 350 degrees F and line baking sheets with parchment paper.

Place the butter and sugar together in a large mixing bowl and beat on medium-high speed until very pale and fluffy (3 to 5 minutes).

Add the egg, and beat on medium-high speed until incorporated.

Stir in the egg yolk and vanilla extract until combined.

Scrape the bottom and sides of the bowl with a silicone spatula, then add the flour, cornstarch, baking soda, and salt, stirring together until incorporated.

Stir in the white chocolate chips and macadamia nuts.

Use a 1.5-tablespoon cookie scoop to drop balls of cookie dough on the prepared baking sheets, allowing two inches in between for spreading.

Flatten the balls slightly with the palm of your hand.

Bake the cookies (one sheet at a time) for 8 to 12 minutes, or until the edges are set and just barely beginning to turn golden. (Take care not to over-bake, or the cookies will not be as soft.)

Cool on the baking sheet and transfer to an airtight container for storage.