Confusingly, if you say 'I'll make cake in 30 minutes' it can have both meanings. It can mean that I'll start making it in 30 minutes from now ( 'tra 30 minuti') , or it can mean that it'll only take me 30 minutes to make it ( 'in 30 minuti'). This might seem odd when you see it written down, but in reality there is never actually any confusion. The stress, intonation and other non-linguistic features of the way you'd say this would be different, and so the meaning would always be clear.

Su Ki is correct. I would like to add that there are additional options, which I sometimes use, especially when speaking with non natives: it takes me 30 minutes to make a pizza. -- in 30 minuti " I will make a pizza 30 minutes from now" -- tra 30 minuti. Notice that English usually uses a future tense for this.