Do you write the premise first, or discover it after the jokes?

I keep ending up with three or four lines that make me laugh but no clean answer to “what is this bit actually about?” Part of me thinks I should force myself to write the premise in one sentence before I keep going. The other part thinks that’s how I turn something funny into homework. When you’re building a bit, do you want the premise clear early, or are you happy to let the jokes tell you what the premise was afterwards?