Toast the macadamia nuts in a dry skillet over medium-low heat for 4-5 minutes, stirring often, until golden and fragrant. Transfer to a plate and let them cool.
In a large bowl, beat the butter, brown sugar and granulated sugar with a mixer on medium speed for 2 minutes, until light and fluffy. Beat in the milk and vanilla until combined.
In a separate bowl, whisk together the flour, cornstarch, baking soda and salt. Add to the butter mixture and mix on low just until combined.Fold in the white chocolate chips and toasted macadamia nuts with a spatula.
Cover and chill the dough for 30 minutes so the cookies bake up thick and chewy. Near the end, preheat the oven to 350°F and line two baking sheets with parchment paper.
Scoop the dough into 2 tbsp mounds and place them 2 inches apart on the prepared baking sheets.
Bake for 10-12 minutes, until the edges are golden and the centers look pale and slightly underdone. They will continue to set as they cool.
Cool on the baking sheet for 10 minutes, then transfer to a wire rack. Makes about 16 cookies.