A favorite of children the world over, these are also at the top of Brad's cookie request list. The KEY to getting that funfetti cake batter flavor is CLEAR vanilla, and the key to softness is room temperature butter and cream cheese, and under-baking them a bit.
Combine butter, cream cheese and both sugars in the bowl of a stand mixer with paddle attachment, or combine by hand, and beat until creamed and a little fluffy.
Add both vanillas and egg and beat on low until incorporated.
Add flour, baking soda, salt and cream of tartar to wet ingredients and mix until combined.
Pour in sprinkles and mix on low until they are well dispersed.
Scoop dough onto a parchment-lined cookie sheet, with each scoop measuring approximately 3 tablespoons.
Chill the balls of dough for about 10 minutes on the cookie sheet in the fridge.
Bake for approximately 10 minutes until the edges are set. The cookies will still be light in color and will have just started to crack. Under-baking these a little is what keeps them nice and soft. (IF you prefer a crunchy cookie you can keep baking until golden brown, but keep a close eye on them).
Cool on the a cooling rack and store in an airtight container at room temperature, ideally while cookies are still slightly warm.
Notes
If you need to speed up the butter-softening process, put the stick in the microwave at LOW power for 5-10 seconds at a time until the butter is soft but NOT melted. It's better for the butter to be a little cold than a little too warm, as it will continue to soften as you work with it. Also, make sure your butter is not in an aluminum wrapper when you put it in the microwave!While this is not written as a vegan recipe, I am confident than exchanging the butter and egg for vegan substitutes would work just fine. If you try this please leave a comment and let me know how it goes! Tip: putting a piece of bread in your airtight container with the cookies can help them stay softer for longer.