Whisk together the All-purpose flour, cake flour, cornstarch, baking soda, baking powder, and salt together in a large bowl. Set aside.
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.
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.
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.
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!
Let the cookies rest: If your cookies are turning out too gooey, let them rest longer. I recommend at least 15 minutes on the cookie sheet and then transfer to a cooling rack to cool completely. They are even better if you let them sit and enjoy the next day. Your cookies aren't ruined, just let them rest!
These cookies stay soft for several days when properly stored in an airtight container or baggie.
You can make cookies smaller. Adjust baking time as needed.
You can also freeze them in a freezer safe baggie. Let them cool completely before freezing.
I personally haven't used a hand mixer for this recipe but I would assume they would turn out just fine.
If your cookies are coming out flat, here are some tips: Make sure you're using cold butter. When forming the dough into balls make sure it's compact. You can also chill the batter in the fridge for 30-60 minutes to see if that helps.
If you don't have cake flour, you can use All-Purpose Flour. They will turn out just fine!