OH my goodness I was just looking for the same thing!! My mare's birthday is the 13th, too, and my colt will be a year old on the 31st. I wanted to make cakes, but everything is for cookies or sweet feed treats, and they already get sweet feed anyway :P
I did see something that called for those rice cakes that Quaker Oats makes (only brand I could think of) covered with molasses and sprinkled with grated carrots and what not-- I was thinking about expanding on that, using the rice cake as a base. Though IDK if those are really horse safe.