Both are possible, but it is not a case of being formal or informal. If you say 'a piece of', you mean one section of something larger. For example, if you have one large birthday cake which you cut into slices, you would offer each of your guests 'a piece of cake' or 'some cake' (uncountable). But if you have a plate full of small individual cupcakes, each person would take 'a cake', or several 'cakes' (plural). It's as simple as that. The same goes for chocolate, for example. If you are handing round a box of little individual chocolates, these are countable, and you'd offer people 'a chocolate' or a couple of 'chocolates' (plural). But if you have a big bar of chocolate which you need to break into squares to share around, you'd say 'a piece of chocolate' or 'some chocolate' (uncountable). As for 'candy', both forms, 'a candy' and 'a piece of candy' are possible. This is US English, so I'm not 100% sure, but I believe that the uncountable form is more common nowadays and the countable form is a little outdated. What I do know, however, is that it's the grammar that's important - you can use either form, providing the grammar of the sentence makes sense. For example: How much candy is there in the box? Right How many candies are there in the box? Right What you can't say is 'How many candy' or 'How much candies' or 'Some piece of candy' , for example. The singular/plural elements of the sentence have to match up - that's what matters most.

"a piece" is the part that is countable. Usually objects that usually come in a set or in packages of more than one, we count as "pieces." You may hear "a candy," depending on the person and where they come from, but I don't think you'd hear the word candy counted: for example, one candy, two candies, etc. However, you would hear: one piece of candy, two pieces of candy, etc. You can also use the word "piece" for objects that can be divided into smaller parts, like cake: one piece of cake, two pieces of cake, etc. However, if you say one cake, two cakes, etc; then you would be talking about counting whole cakes and not the pieces that you can cut from them. Does this make sense? I hope my explanation helped. If you have further questions please feel free to ask.