The first thing that popped into my head, when I thought of 55, was a speed limit sign.
55th Anniversary Cake by Tracker-Backer, on Flickr
Naturally, I'd already started to frost the entire cake when I remembered that I was going to leave a section unfrosted for my dad. Scraping it down to "just very lightly frosted" kind of made a mess of that section; oh, well.
As a non-chocolate companion, I tried baking a DH "classic white" with whole eggs instead of just egg whites, in hopes of getting a tiny bit more volume (that mix was a bit anemic on volume even before they made it smaller). No decoration to speak of, so I didn't bother to photograph it.
Very nice James! Is the speed limit sign fondant? I bet they loved it.
Edible image. Found out this past week that the local cake supply uses DecoPac edible inks, possibly their media as well.
Even though there's enough translucency in the edible image media that you can see where there were air bubbles trapped under the image, there's no sign of the chocolate frosting visibly diffusing into the media, even 30 hours later.
Image was rasterized in GIMP, from an SVG on Wikimedia Commons (somebody supplied Wikimedia Commons with SVGs of every official speed limit sign you're ever likely to need), then I added the graffiti ("WHAT") in a text layer (Comic Sans is good for something), then did a select-by-color of the text layer, feathered the selection, and painted it red, to give it a spray-painted look.