I sometimes like to write the documentation for the module, to varying degrees of completeness, before I write any code. I find that for infrastructure modules it really helps to make sure that you're writing an API that serves the API user, instead of serving the API designer, because you discover right away if you're writing an idiotic API.

If you want to smear a thin patina of faddishness over it, you could declare the documentation is a form of TDD, except that we're testing something that automated testing simply can't, which is the effective usability of the API from a human point of view. But then, I'm not too worried about yon thin patina of faddishness. Sometimes it's the right tool for the job, often it isn't.