I think that hazelnuts go perfectly but we can't overlook that walnuts would also make a fabulous pairing. You want to make sure that you use a nut that is on the softer side and with a "buttery" texture if that makes sense. Nuts like macadamia nuts would be a perfect example of this but those are very high priced so walnuts and hazelnuts are good affordable options. Harder nuts like almonds and pecans may taste great but will need to be slivered so that they aren't too hard to bite through.