In the bowl of a stand mixer, affixed with the paddle attachment, add the butter and both sugars. Mix on medium-high speed until light and fluffy. Add the eggs yolks and mix until incorporated. Add the butter extract and imitation vanilla. Mix until combined.

Turn the mixer to low speed and add the flour, salt, baking soda, baking powder and cornstarch. Mix until barely combined.

Remove the bowl from the mixer and use a rubber scraper to ensure all the ingredients are incorporated. Fold in the rainbow chips and sprinkles.

Line three baking sheets with parchment paper. Scoop standard sized cookie dough balls and place on the baking sheets. Refrigerate for one hour.

Once dough has chilled, preheat oven to 350-degrees and bake cookies, one baking sheet at a time for 9-11 minutes. Cookies may appear underdone, but they continue to set on the baking sheet. Allow cookies to cool and set on the cookie sheet for 30-minutes before transferring to a wire rack to finish cooling. Repeat process with remaining baking sheets of cookie dough balls.