This is my favorite kind of ghost. I have seen people do this technique with chocolate syrup (which you certainly can do by painting the inside of the glass with it) but I do it a little differently. I like to use the circular dark chocolate candy melts to get this effect. It makes the shape of the mouth and eyes more defined. This marshmallow trifle is very rich (and messy). Below is the recipe and tutorial for this dessert.
What you'll need:
Circular Dark Chocolate Candy Melts
Marshmallow Cream Recipe:
1. Freeze or chill your mug so it is cold. Then place a candy melt and put it point side down on a microwave safe plate. Microwave for about 7 seconds. You want the melt to be "melty" enough on the bottom to leave a little chocolate on the plate, but still hold a shape. On the inside of the mug, place the chocolate where the eye should go, melted side against the glass.
2. Repeat step 1 for the other eye. For the mouth, do the same thing but slide the melt down so chocolate covers a wider surface, making the mouth almost twice as large. (You have to work quickly though, because the chilled glass will make the chocolate harden very quickly.) When you're not working with the mug, keep it in the freezer or fridge.
3. Fill the bottom of the mug with white cake peices. Next, add a layer of Marshmallow cream (it should be warm enough to be pourable but not so hot that it messes with the chocolate face). Reapeat the layers, topping it with whipped cream. You'll want the whipped cream to be extra thick so it will give you that ghost top shape. Putting it in a ziploc bag with the tip cut off (make it a large tip) and then piping it onto the trifle will help.
Chill your dessert until ready to eat.