You can get started on pinning down bus information at the website Rome2Rio.com, which tells me the company Tiemme runs hourly buses from Florence to Colle di Val d'Elsa, and that the run takes about an hour. Your target town seems to be a stop on the route to Siena, which probably explains the frequency of the buses. Information like this must always be verified.
This seems to be the outbound schedule: http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/81a9b01138449e935a383001e63fade1.pdf
Colle di Val d'Elsa is much closer (at least time-wise) to Siena than it is to Florence, so that would be another possible base for you before and/or after the proposed cooking class.
It appears that most of the buses do not run on Sunday.
In my experience, the buses stick to the schedules quite well, but you must be sure you're looking at the right schedule and interpreting all those codes correctly. I always try to check with the Tourist Office for current information, and if it's convenient to go by the bus station, I do that too. Be aware, however, that English will often not be spoken at the bus station ticket windows. I confess that I have never had a time-sensitive appointment at the other end of a bus ride. Italy isn't Switzerland.
