Howdy all. I've seen a few recipes calling for both, and while I've only tried one recipe at this point (I made one batch with sour cream and one batch with cream cheese), I still I don't know what either ingredient is supposed to add to cookies in terms of texture or taste. So, I'm really hoping some of you culinary masters/mistresses will help enlighten this novice.
Why use cream cheese or sour cream in cookies?
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Fat, moisture, flavor. Really, it's the same ideas as using butter, or shortening. Butter has it's own flavor, shortening doesn't and so on.
Because of the different moisture of each of these products, you'll also get different behavior in baking for a cookie. Shortening would spread the least as it has no water content to speak of. Butter spreads more. Cream cheese even more, sour cream quite a bit, all other things being equal. (Which they won't really be, but that's part of how you build a cookie recipe.)
So a cookie will tend to get flatter, thinner and wider with the sour cream as the only fat than any of the other choices. Of course, you can use some other fats to thicken it up some more and use the sour cream mostly as a flavor accent. Specifics depend on the recipe.