Batman is in trouble from the Joker, and a large yellow beast. Robin, just a boy at the time, touches a liquid mirror (like in the matrix when Neo touches one), becoming an adult-ish Robin, no longer just an ordinary boy.
It's from the 80s/early 90s as far as I can remember. Maybe even a different continuity from the standard DC Batman.
Also, i remember it was from one of those types of issues, where batman had long ears and was very dracula-like (such as seen in the issues of 400-500s).
EDIT: The comic itself had many advertisements, the first page had a Chuck Norris type guy shattering a brick wall by kicking on the first few pages.