The squishy and sugary treat was developed by Vat9, but is currently sold out. Here's the description from their website:

Whether you try to think of the most harmless thing, something that could never, ever possibly destroy you or you try to think of a giant terrorizing destructor that can stomp buildings to rubble, you're thinking of the same thing—the Stay Puft Marshmallow Man. To save the world, you must risk being annihilated by crossing the streams of your proton pack... or just devour every marshmallow morsel of his delicious body.

Here’s the scene where the Stay Puft Marshmallow Man made his first appearance:

Credit: Vat19 via YouTube