Get Ready to Bake! Preheat your oven to 350°F (175°C) if you're baking these delicious cookies right away. Line your baking sheets with parchment paper for easy cleanup.
Whisk Dry Ingredients: In a medium bowl, gently whisk together the flour, cornstarch, baking soda, baking powder, and fine sea salt. This ensures all the dry goodness is evenly distributed. Set this mixture aside for a moment.
Cream Butter & Sugars: In a large bowl, grab your electric mixer! Beat the cool room-temperature butter with the brown sugar and granulated sugar until the mixture is beautifully light and fluffy. This usually takes about 2 to 3 minutes.
Add Eggs & Vanilla: Now, incorporate the eggs and egg yolk, adding them one at a time and beating well after each addition to ensure everything is perfectly combined. Stir in the pure vanilla extract for that lovely aroma.
Combine Wet & Dry: Gradually add your whisked flour mixture to the wet ingredients, beating on low speed until everything is just combined. Be careful not to overmix; we want tender cookies!
Fold in Goodies: With a rubber spatula, gently fold in the sweet white chocolate chips and the wholesome macadamia nuts until they're evenly spread throughout the dough.
Chill (Optional for Best Flavor!): For an even richer flavor and a delightfully chewier cookie, cover your dough in plastic wrap and refrigerate it for at least 24 hours, up to 72 hours. If chilling, let it sit at room temperature for a bit until it's just soft enough to scoop. If you're in a hurry, you can skip this step!
Scoop & Bake: Use a large (3-tablespoon) cookie scoop to drop uniform balls of dough onto your prepared baking sheets, making sure to leave about 2.5 inches between each for optimal spreading.
Golden Perfection: Bake your cookies for 11-12 minutes, or until their edges are wonderfully golden brown.
Cool & Enjoy: Let the cookies cool on the baking sheet for 5 minutes before carefully transferring them to a wire rack to cool completely.
Store Fresh: If you have any cookies left (a rare occurrence!), store them in an airtight container at room temperature for up to 3 days to keep them perfectly fresh and soft.