Place the marshmallows and butter in a very large, microwave safe mixing bowl.
Microwave in 15-second blasts, stirring between each burst until melted and smooth. Be careful, it can get hot.
Add the green food colouring. Mix until the colour is evenly distributed. If your mix starts to set, you can pop it back in the microwave for another short burst.
Add the rice crispies. Mix until evenly coated.
Line a muffin tray with cupcake cases.
Pile the mixture into each case. I find it easiest to use my hands to do this. Wet them just slightly under the tap to prevent your hands from sticking and wash them from time to time when the mixture starts to stick.
Push a chocolate ball onto the top of each crispy cake.
Use your black writing icing pen or black fondant to add a black dot to the top of each chocolate - these are the pupils.
The cakes will only take about 15 minutes to set, then you can serve and enjoy!
Store these crispy cakes in a single layer in a sealed container somewhere cool and dry. Properly stored, they should keep for 2 days. If frozen in the same way, they should keep for up to 6 weeks. They'll defrost in 15 minutes at room temperature.