Combine flour, cornstarch, and salt into a medium mixing bowl and whisk until combined.
Dissolve baking soda and warm water in a small bowl, set aside.
Using a stand mixer or hand mixer with paddle attachment, cream the butter, granulated sugar and brown sugar until light and fluffy (3-5 minutes on medium speed). You will know its done when the color has lightened and is fluffy.
Add the vanilla, cake batter extract and egg until and mix until fully incorporated.
Combine the baking soda/water mixture just until incorporated and beat on medium for 20-30 seconds.
Add the dry Ingredients (flour, cornstarch, salt) and mix until just combined. Stop the mixer when you still see some flour on the side of the bowl and add the sprinkles and fold with a spatula until incorporated. This should only take 10 seconds- make sure to not over-mix the batter.
*Optional. Place cookie dough in the fridge for 30 minutes while the oven comes to temp (not a requirement but simply enhances the flavor and will yield a softer cookie inside since it takes longer to bake colder dough).
Preheat oven to 350° F and line a baking sheet with parchment paper.
Use a tablespoon or cookie scoop to make 12-15 cookies, place 2-3 inches apart on the baking sheet and bake for 11-15 minutes (depending on your oven and how cold the dough is). Tip: start by scooping 1 cookie to test at the minimum time- 11 minutes and adjust accordingly. Take out of the oven as the edges start turning a golden brown. The center may still seem uncooked but it will continue to bake once out of the oven, during cooling.
Transfer to a cooling rack and enjoy!