In KitchenAid, cream together butter and cream cheese until well-combined.
1 cup butter, 4 oz cream cheese
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.
1 cup sugar, ½ cup brown sugar
Add egg and beat until combined.
1 large egg
Stir in vanilla extract.
1 tsp vanilla extract
In separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, and salt.
3 ½ cups all-purpose flour, 1 tsp cornstarch, 1 tsp baking soda, ¼ tsp 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.
1 ½ cups premium white chocolate chips
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).
¼ cup colored sprinkles
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.