No explanation needed, but kaffet or kaffen?
ett kaffe - kaffet
If you order a cup of coffee in a cafe, you can ask for "en kaffe" (en kopp kaffe) though.
Likewise, if you order coffee beans in a shop you have to use "ett"
If you order coffee beans, you either refer to them as a plural, or you use another noun such as ett paket kaffebönor.