Tough call. If you enter it in category 23, the judges will EXPECT that it will be something significantly different from a traditional robust porter. If you state in your entry registration that chocolate is what makes it different, it had better have a recognizable and substantive chocolate flavour that improves upon a regular robust porter.
If the chocolate flavour is rather subdued and doesn't distract from the expected characteristics of a robust porter as described in the BJCP guidelines, then it should be entered as a robust porter (12B).
I have seen a lot of beers tossed from category 23 because the judges don't think it was 'special' enough.