In KitchenAid, cream together butter and cream cheese until well-combined.
Add sugars and beat on medium-high speed for 2 minutes, until light and fluffy. Pause mid-way through to scrape down the sides of the bowl.
Add egg and beat until combined.
Stir in vanilla extract.
In separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, and salt.
With KitchenAid on low speed, gradually add flour mixture until completely combined.
Stir in white chocolate chips (low-speed), and pause to scrape down the sides of the bowl.
Add colored sprinkles and stir until just combined (by hand is preferable, but the dough is quite sticky and thick, so you may mix briefly on low-speed).
Cover and transfer to refrigerator to chill for 30 minutes. Meanwhile, preheat oven to 350F and line cookie sheets with parchment paper.
Once cookie dough has chilled, remove from fridge and scoop/roll into 1 ½" balls. Place on parchment paper and slightly flatten with your hand or with the bottom of a clean glass. Add additional sprinkles if desired.
Bake for 8-10 minutes, remove from oven and allow to cool for at least 10 minutes on cookie sheet before transferring to cooling rack to cool completely.