Production I.G used to be my favorite anime production house (not even Studio 4C could touch them in their prime). Grown out of the Headgear collaborative (the production team behind Patlabor 2 and Oshii's prior home), I.G produced classics like Kokaku Kidotai, Innocence and Dead Leaves. Once a synonym for quality Japanese animation features, nowadays they keep on spewing out mediocre series and movies just to keep their head above water.
Tokyo Marble Chocolate is a 2-part OAV from I.G's transition period (2005-2007). It's already a lot softer in tone and style than their previous output (more in line with the kind of series Gainax used to produce), yet the unique artstyle and the Shinkai-inspired romance (Kotonoha no Niwa, Byosoku 5 Senchimetoru) make for a sweet and amusing little diversion.
The story revolves around a boy and a girl hooking up. Both are not very successful when it comes to maintaining relationships. Even though they seem to like each other a lot, they can't really commit fully to their blossoming love. The OAV is split into 2 distinct segments, each segment following one character going through the one decisive day that will shape their future as a couple.
While mostly straight-up romance, there is at least one thing setting it apart from other, more typical romances. Shiotani includes an extra character: a rather unique and feisty looking mini-donkey. Through this character some extra comedy is brought into the OAV, while at the same time making it a bit more attractive to the regular anime crowd. I'm not really sure it helped the OAV as a whole though, as it does contrast rather strongly with the romantic atmosphere.
Clocking in at around 50 minutes in total, Tokyo Marble Chocolate is a pretty safe bet for people who're into Shinkai's romances. It may be a more commercial move for I.G, but the artstyle and soundtrack make sure that there is still plenty to enjoy.