In a medium bowl, combine the dry ingredients (flour, baking soda, salt and cornstarch). Whisk together and set aside.
Using a stand mixer or a hand-held mixer, cream together the room temperature butter and sugars (brown and white) on medium (4) speed for 1-2 minutes. Scrape down the bowl with a rubber spatula.
Add the egg and vanilla to the butter mixture. Beat on low-medium speed (2) for about 30 seconds and then increase speed to medium (4) for another 30 seconds. Scrape down the bowl.
Slowly add the dry ingredients to the butter mixture on low speed (1) until just incorporated.
On low speed add the white chocolate chunks and cranberries. Mix until just incorporated.
Chill in the fridge for a minimum of 30 minutes (preferably 1-2 hours).
Preheat oven to 325 degrees (160 c).
Scoop the dough into balls (about 2 tablespoons) onto a parchment lined baking sheet. Bake for 9-10 minutes. Remove from the oven once you see light golden brown edges. Leave them on the baking sheet for another 5 minutes to finish cooking and then place on a cooling rack to finish cooling.