In the bowl of a stand mixer, cream together the softened butter and sugars on medium-high speed until they’re whipped and creamy. Next, add in the egg and vanilla, and beat on high speed and mix until evenly incorporated.
In a separate bowl, stir together the flour, cornstarch, baking soda and salt. Working a little at a time, mix the dry ingredients into the wet batter. (The batter should be pretty thick, and that’s ok) Use a spatula to gently fold in the white chocolate chips and the dried cranberries. Cover the dough tightly with plastic wrap, or transfer to a Tupperware and seal, and chill for at least 2 hours, or up to 2 days. Do not skip this step!
Remove the dough from the refrigerator about 10 minutes prior to baking to allow it to come almost to room temp, if you let it chill overnight. It may take up to 30 minutes. While the dough softens, prep two baking sheets with parchment paper or silicone baking mats, and set aside.
Scoop about 1 tablespoon of dough and roll, between your hands, into a ball. If the dough is crumbly, no worries, it will smooth right out when you roll it. Put the dough balls onto a non-stick baking sheet, about 1-1/2 – 2 inches apart from each other, to allow for spreading. Bake at 350 degrees for 8-10 minutes, or until just barely golden brown around the edges. They will look extremely soft when you remove them from the oven. Let the cookies rest for 5 minutes or so, until you transfer them to cooling rack to finish cooling completely.