For these cupcakes I used white chocolate that I melted down and poured into a chocolate ghost mold. I chilled it in the fridge for about 10-15 minutes and popped them out onto a towl. I added some colored sprinkles to the frosting so that the ghosts would show up better and even brushed it with some white satin luster dust to make it shine a little.
The ghosts could also be made with fondant as well. You can either grease or corn starch the chocolate mold and press the fondant in it. If you have troubles getting it out with out stretching the shape then freeze in the freezer for a few minutes then pop them out. You could also make the ghost by free hand as well using gumpaste tools to make the creases in the body and eyes. If you make them out of fondant I would let them sit out for 3-4 days so they can harden or they will be limp on top of the frosting and sag.