Preheat oven to 340°F (170°C). If your oven can’t be set to that exact temperature, use 350°F (175°C) and start checking the cookies early. Line a baking sheet with parchment paper to prevent sticking.
In a large bowl, cream the softened butter, brown sugar, and granulated sugar until light and fluffy. This step helps create a softer cookie.
¾ cup (170 g) unsalted butter, ¾ cup (150 g) packed light brown sugar, ¼ cup (50 g) granulated sugar
Add the egg, extra yolk, and vanilla extract. Mix until smooth and fully combined for a richer, softer center.
1 large egg, 1 egg yolk, 1½ tsps (7.5 ml) vanilla extract
In a separate bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt.
2 cups (240 g) all-purpose flour, 1 tsp (3 g) cornstarch, ¾ tsp (4 g) baking soda, ½ tsp (2 g) baking powder, ½ tsp (3 g) table salt
Add the dry ingredients to the wet ingredients in thirds, mixing just until combined each time. Do not overmix or the cookies will turn dense instead of soft.
Fold in the white chocolate chips and chopped macadamia nuts until evenly distributed.
1 cup (170 g) white chocolate chips, 1 cup (135 g) roasted and salted macadamia nuts
Scoop the dough using a #16 cookie scoop (2 ounces / 56 g each) and place on the prepared baking sheet, spacing the cookies well apart so they bake evenly.
Bake for 11–13 minutes, until the edges are lightly golden and the centers look slightly underbaked. They will finish setting as they cool, which keeps the centers soft.
Immediately after baking, be ready with an upside-down glass to use over the top of the cookie to gently shape them into a more uniform round if desired. This is key if you want perfectly round shaped cookies.
Let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely so they set without overbaking.