Grease and line an 8x8 inch square pan with a double layer of cling wrap.
Place marshmallow fluff and butter into a large saucepan over a low heat. Stir continuously until all the butter is melted and the mixture is smooth and well combined.
Add in the green food coloring a few drops at a time. Stir until you’re happy with the color.
Mix in the rice krispies, adding 1 cup at a time, until they are all coated in the green marshmallow mixture.
Tip the mixture into your prepared pan and press into an even layer with a spatula.
Leave to set for 30 minutes in the fridge.
Once chilled, transfer the krispie treats from the pan to a cutting board and cut into 2.5 x 2 inch rectangles.
Place a wire rack over kitchen paper and get ready to decorate!
Place the candy coating into a medium sized shallow bowl and place in the microwave in 30 second increments, stirring in between until all the coating is melted.
Take ¼ of the mixture and place in a small Ziploc bag. Leave to one side. Dip the top of each cut krispie treat into the bowl of melted candy coating. You want it to come ¼ - ½ inch of the way down the krispie treat; this is his hair line. Repeat with all of your krispie treats and then place them on the wire rack to set.
Use one of the toothpicks to place a little of the extra candy coating onto the back of your candy eyes and stick them to the front of your krispie treats, about ½ inch from the “hair”.
Snip a tiny hole into the end of your Ziploc bag and pipe a line ½ inch away from the candy eyes across the krispie treat. Then pipe a line perpendicular to the line across the treat at either end. Pipe two more along the line to create his mouth.
You can also pipe a zig zag line for his mouth, whichever you prefer.
Poke the remaining toothpicks through the side of each krispie treat in line with the mouth. Place a mini marshmallow on each end of the toothpick.
Repeat until all of your krispie treats are decorated, then leave to set for 20-25 minutes.
Frankenstein Rice Krispie Treats will keep in an airtight container, at room temperature for 3 days, or in the fridge for 5 days.