Lightly spray 8" square glass baking disk with non stick spray. Wipe off excess.
Whisk together water and gelatin in microwave safe bowl.
Microwave for 1 minute.
Whisk contents well, add marshmallows and stir.
Microwave for 30 seconds and stir until marshmallows are dissolved. If they do not completely dissolve microwave another 15 seconds.
Pour contents into prepared disk and refrigerate for 30-45 minutes.
Use a butter knife to loosen around edges. Roll up and place seam side down on cutting board.
Use dental floss or twine to cut into slices by sliding under roll, wrapping over top and pulling ends.
Refrigerate slices until ready to serve.