During a recent trip, I noticed that many flight attendants mispronounce the word fasten (as in “fasten your seatbelt“), even though they use this word very often (I’ll come back to “often” soon). In contrast to the word fast, the t in fasten is not pronounced. Hence, fasten is pronounced as [ˈfɑːsn] (British English) or as [ˈfæsn] (American English); optional pronunciations are [ˈfɑːsən] and [ˈfæsən]. You can listen to the pronunciation here.
The same logic applies to words such as hasten, chasten, christen, glisten, listen, and moisten. For other pronunciation “rules” regarding the letter t, have a look at this previous post. In contrast, the t in often can be pronounced or not pronounced, see here. (Note that the t follows an f rather than an s as in the above examples). According to the Longman Pronunciation Dictionary, a survey revealed that about 75% of the people asked pronounce often without a t, and 25% pronounce it with a t.