If you're up to trying local specialties, you can find delicious treats at all places called "pasticceria" - those are pastry shops.
There are lots of cakes with cream and chocolate, you will be hard pressed to decide what to try. These are not traditional "cakes" in general, not like birthday cakes you find in the US, for example. They are not the type of cakes Italians eat for birthdays, they eat pastry type cakes which are the ones you'll see. You'll find pasticcerias everywhere, so not sure what to recommend, I wouldn't suggest one over the other unless you tell me which area in Florence you are staying in and something comes up to mind as being the area.