What is the ruling for breaking a recommended fast i.e. of Rajab or Shaban intentionally? What’s the kaffara?

Mustahab fast can be broken anytime and there is no kaffara for that. But if the Fast was wajib then you can fast 60 days for one fast broken deliberately. If you are not capable health wise to fast 60 days then feed 60 poor people or give one mudd (=3/4kg) of foid- stuff, like wheat, barleyor bread etc to each of them. If you cant feed 60 people then seej divine forgiveness from Allah (swt). And Obligatory precaution is that you should give kaffarra as and when you are capable to do so.