Combine Dry Ingredients: Whisk the all-purpose flour, baking soda, and salt together in a medium bowl. Set the mixture aside.
Cream Butter and Sugars: Place the room-temperature butter, light brown sugar, and granulated sugar in a large mixer bowl. Beat on medium speed until the mixture becomes noticeably lighter in color and fluffy. Scrape down the sides of the bowl several times during mixing as needed. Add Egg and Egg Yolk: Add the large egg, followed by the egg yolk, mixing until each addition is thoroughly incorporated before adding the next.
Add Vanilla: Mix in the vanilla extract until evenly combined.
Combine Dough: Add the dry ingredients to the butter mixture and mix just until the dough comes together. Avoid overmixing.
Adjust Dough Consistency: Add 1 teaspoon of milk and mix on low speed just until incorporated. Use a rubber spatula to finish bringing the dough together. Add the second teaspoon of milk only if it is really needed. Avoid adding excess milk, as too much liquid can cause the cookies to spread more than desired. Add Chocolate and Macadamias: Stir the white chocolate chips and chopped roasted salted macadamia nuts into the dough until they are distributed throughout.
Shape Dough: Portion the dough into balls approximately 2 tablespoons in size, using all of the dough.
Chill Cookie Dough: Refrigerate the shaped cookie dough for at least 2–3 hours or overnight. After chilling, let the dough sit at room temperature for about 1 hour so it can come closer to room temperature before baking.
Prepare Oven and Baking Sheets: Preheat the oven to 350°F (180°C). Line the baking sheets with parchment paper or silicone baking mats.
Bake Cookies: Arrange the prepared dough on the lined baking sheets and bake for 8–9 minutes. Remove the cookies from the oven as soon as the edges begin to turn golden. Do not overbake. The cookies will be slightly puffy when first removed from the oven and will settle somewhat as they cool. Cool Cookies: Leave the cookies on the baking sheets for 4–5 minutes after removing them from the oven. If they have spread slightly more than desired, gently push the edges inward with a spoon while they are still warm. Transfer the cookies to a wire rack and allow them to cool completely.