You cannot use a QS credit, you MAY be able to use a snack credit. It's been a year or so since I've seen reports of being able to get a single scoop here, so I'd definitely ask, if that is what you want. There have been more recent reports that some of the packaged stuff in the store is available for a snack credit, but again, you will have to ask.
You can use snack credits. Although in my opinion it's not a great use of a snack credit. I believe you can only get a small scoop of ice cream, no sundae's, specialty things etc. You can get much better use out of a snack credit at other ice cream places throughout Disney parks/resorts.
Hmmm...I'm surprised you were able to get them in a waffle cone. We've asked several times over the last few years and have always been told no in regards to using it as a snack credit (could only get sugar cones).
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