Ask professor if she is 'still' willing to write me a LoR?
There's no reason why you can't ask:

Dear Prof. X:
I was a student of yours for class Y in term Z. While I was still an undergraduate, we had talked about the possibility of you writing a letter of recommendation for me. At the time, I did not end up applying to graduate schools at the time, but will be doing so . Would you be willing to write a letter for me now? I can provide you with an updated résumé/CV.

with appropriate modifications.
You can definitely mention that she previously agreed to write a letter—you might be refreshing her memory, in case she gets a number of requests for such letters!