Ruggelach are one of my favorite cookies. Would non fat yogurt work as a replacement for the sour cream?
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I've never tried it, but it's worth a shot. I'd make sure to use a thicker Greek-style yogurt, or drain a standard yogurt, to end up with a sour cream-like consistency.
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I agree with deensiebat. The sour cream, or yogurt, acts to tenderize the dough. Non-fat anything isn't going to give you the same result. But, heck, it's all a science experiment, so try it and see. If you don't like them, there's your answer.
I've used thick Greek yogurt in place of sour cream in scone dough and it worked perfectly. I imagine the result would be much the same in Ruggelach. Good luck!