Cook your sugar cookies and try to leave some uneven edges. Place on a cooling rack to cool completely.
Whisk together the ingredients needed for the white frosting until the frosting is smooth and runny enough that it drips off a whisk easily.
Slightly melt the marshmallows by placing them on a silicone mat or plate and sticking them in the microwave for about 15-20 seconds, or until they puff up. It's a good idea to spray the plate with nonstick cooking spray to avoid a sticky mess. Make sure there's at least an inch or two between them as they will expand.
Spray a rubber scraper and use that to quickly transfer the melted marshmallows off the mat and onto your cookies.
Grab a spoonful of white frosting and drizzle it over the marshmallow topped cookie. Try to completely cover the marshmallow so that it's hidden. Some of the frosting will drip off of the cookie, this is okay.
Whisk together the ingredients for the back frosting and transfer it to a Ziplock bag. Snip the corner end and pipe the frosting onto the cookie. Make two eyes and a large mouth. Let the frosting dry and pat down any lumps with your finger. Voila! 3-D Ghost Cookies!