"Ga i baned o siocled?"
Translation:May I have a cup of chocolate?
To specify "a piece" I might try Ga i ddarn o siocled?
The baned (cup) here hints that this is requesting something to drink rather than something solid. But I suppose one could ask for a cup made of (a wneir o) chocolate... I think that would be Ga i baned a wneir o siocled?