"My coffee cup is empty."
Translation:Mia taso de kafo estas malplena.
I don't think that would be the only interpretation. The parentheses around 'made of' seem to suggest that 'made of' is just one possible reading. After all, a 'reta vortaro' is not a 'dictionary made of net', just an internet dictionary. The Slavic languages that Zamenhof was familiar with are very liberal, so to speak, in making adjectives out of nouns and then using these in all sorts of meanings. Still, perhaps the Esperanto community has come to use 'taso de kafo' instead of 'kafa taso'. That I don't know.
I like kaftaso best, with kaf-taso being the same thing but stressing the compound, but why is "kafotaso" valid? That literally means "coffee-noun-cup-noun", and out loud could be taken as some talking really fast about his cup of coffee. Is it just because people are really used to saying words with their part-of-speech endings and dont like dropping them as roots?