This leaves out the "not too rich part" of your request, but one of my favorites is to make a regular chocolate cake. Let it cool slightly, then poke holes in it with the end of a wooden spoon. Pour a jar of Hot fudge ( heated) into the holes and across the top of the cake. Cool competely. Top with Cool Whip and crumbled candy bar or cookies (Butterfinger, Oreos, Etc)