Preheat the oven to 400°F.
Whisk together the All-purpose flour, cake flour, cornstarch, baking soda, baking powder, and salt together in a large bowl. Set aside.
2 1/2 cups All-Purpose Flour, 1 1/2 cups Cake Flour*, 2 tsp. corn starch, 1 tsp. baking soda, 1 tsp. baking powder, 1 tsp. salt
Place the butter into the bowl of a stand mixer and secure the paddle attachement. Turn the mixer on the lowest setting. Let the mixer cream the butter for ~30 seconds, followed by the white sugar for ~1 minute.
1 cup cold, unsalted butter, cubed, 1 1/4 cup sugar
Then add in the eggs, egg yolks and cake batter extract and blend until mixed. If any of the butter gets stuck on the paddle attachment use a rubber spatula to knock it off.
2 large eggs, 2 large egg yolks (no whites, just the yolks), 1 tsp. Cake Batter Extract
Gradually add in the flour mixture. I add in about 1/4 cup at a time. Then add in the sprinkles. Start by adding in 1/4 cup sprinkles, and add more as needed (I would recommend no more than 1/2 cup in the dough though since we're rolling them in sprinkles too). Mix until the sprinkles are incorporated. Turn the machine off.
1 cup sprinkles, divided in half
Measure out ~7, 6 oz. cookies. Roll the dough in more rainbow sprinkles in a bowl or shallow dish. Place 4 cookies on 1 cookie sheet and 3 on another. No need to spray the cookie sheet with baking spray.
Bake cookies for 9-11 minutes or until cookies appear done (see recipe notes if your cookies are too gooey!) Let them rest on the cookie sheet for 15 minutes and then transfer them to a cooling rack to cool completely. Enjoy!