A lemon sweet. It's a sweet thing that muggles eat. I like this ???
I understand the concept of role language for portraying different kinds of character. The speaker (professor Dumbledore) is an old man so he ends sentences with じゃ instead of だ for example.
I'm assuming that the ending でな is something like this.
The English translation is "I'm rather fond of them" (actually "it's a muggle sweet I'm rather fond of"). So, I can't see what function でな is performing.
Maybe it's not role language. Maybe it's just すきだ->すきで implying an unfinished sentence, with the sentence ending particle な on the end (a particle I've never got to grips with). I can't see why the translator would do this though.
In either case I can't understand what's going on here.