My sister and I both did this for the same class (several years apart, mind - she's eight and a half years older than me!) in middle school.
We had to do some sort of project, I don't remember exactly what now, for a unit on cells, and sister and I both brought in a homemade sheet cake decorated to look like a cross section of a cell. I don't remember the specifics, but I do remember that at least mine used an Oreo cookie for the nucleus, and gumdrops for something else. Both times, it was a huge hit.