Okay, it's not a Halloween-themed cake, but it was for my sister's birthday, which is on Halloween, so I'm including it. Because I can.
Not much to say about the cake - it was chocolate, of course, and I cut that white piece out of fondant and then painted the Power Puff Girls on there (and the writing and little dots of color) with some gel food coloring mixed with a little water.
Mere was a big fan of the Power Puff Girls. Probably still is.
Anyway, it was a fun one to work on. Cartoons are, at least to me. And painting on white fondant with food coloring is fun, too. The colors are vivid and basically make your edible cartoon look pretty much like the one on paper (or on your computer screen) that you're copying.
I remember I painted the girls on the fondant and let it dry overnight or something. Then all I had do after I frosted the cake was to (gently and carefully) lay the fondant piece on top of the cake. Ta-da!