Stir the milk and ground flaxseed together in a large mixing bowl. Set it aside for now. It will need to sit for at least 5 minutes, to gel.
While the flax mixture sits, whisk the flour, cornstarch, baking soda, cinnamon, and salt together in a large bowl.
Add the butter, sugar, brown sugar, and vanilla to the bowl containing the flax mixture. Using an electric mixer at high speed, cream the ingredients together for one minute, until well mixed and slightly fluffy.
Using the mixer at low speed, begin beating the flour mixture into the butter mixture, adding about a third at a time and completely beating in each addition before adding more.
Use a spoon or spatula to fold the white chocolate chips and macadamia nuts into the dough.
Shape the dough into a large ball and wrap it tightly in plastic. Chill the dough for 2 hours.
Preheat the oven to 350°F and line a couple baking sheets with parchment paper.
Drop the dough in rounded dollops, about 2 tablespoons each, onto the prepared baking sheets, leaving about an inch and a half between them.
Bake the cookies until they've spread out and puffed up a bit, but are still very soft, 10 to 12 minutes — they will seem a bit undercooked, but will firm up as they cool.
Transfer the baking sheets to cooling racks. Let the cookies cool for 5 to 10 minutes on the baking sheets, until they seem set. Remove them from the baking sheets and transfer them directly to the cooling racks to finish cooling.