The act of filling a woman's vagina with marshmallows. Then proceeding to fuck her until they get all melted and gooey, pulling it out and having her suck and/or lick it clean. This is similar to the melted Stay Puft marshmallow man at the end of Ghostbusters, just imagine the dick as the proton gun.
Dude Uno: I did the most awesome thing last night with my girlfriend!
Dude Two: Whats that?
Dude Uno: I gave her the Stay Puft Meltdown!