In a large mixing bowl combine butter and all 3 sugars together, mixing with a electric mixer on medium speed until creamed together. Add in egg, egg yolk, and vanilla extract and continue mixing until thoroughly combined.
Slowly add in flour, cake mix, pudding mix, baking soda, and salt. Continue to mix until everything is combined and cookie dough has a thicker texture.
Add white chocolate chips and sprinkles into the batter, folding until evenly distributed throughout the dough.
Chill cookie dough in the fridge for at least an hour before baking. You can chill dough over night as well but as a minimum time chill for at least an hour.
Preheat oven to 350 degrees. Line a cookie sheet with parchment paper or lightly grease with cooking spray. Scoop small, about 1 to 2 inch cookie dough balls and place them on the baking sheet, about 2 inches apart. Bake for 9-10 minutes. Do not over bake!
Remove cookies from oven and allow to cool for 10 minutes before serving.