Grease a 9x13 baking dish with cooking spray.
Melt the butter in a large sauce pot or Dutch oven on the stovetop.
Stir the vanilla extract and salt into the butter.
Add the mini marshmallows to the melted butter mixture, stirring until the marshmallows have fully melted.
Then add green food coloring until you have the shade of green you want.
Remove the mixture from the heat, and quickly stir in the Rice Krispies cereal.
Spread the Rice Krispies mixture into the greased baking dish and let them cool.
Cut the entire pan of treats into squares or at least, as close to square shaped treats as you can.
Using the tip that comes with or goes on your icing, add hair to your Frankenstein with the black icing. Just do a zig-zag along the hairline, and outline around the top of the head. Then fill in with black icing.
Also, add a small smile or mouth, as well as a piece of stitching to one cheek.
Add candy eyes, setting them in place with a tiny bit of icing on the back.
And that's it! They're ready to serve at your Halloween party!