Begin to melt the butter over low heat in a medium or large saucepan.
Add the marshmallows to the saucepan and stir.
Continue to stir often, until about 75% of the marshmallows are melted.
Remove the pan from the heat and continue to stir, smashing the marshmallows using a rubber spatula or the back of a wooden spoon, until the marshmallows completely melt and become smooth.
Add green food coloring and stir.
If needed continue to add more coloring until you get a really vibrant "grinchy" green color.
Pour the marshmallows over the cereal and toss to combine.
If your cereal treats are not green enough, you can add more drops of green food coloring then mix the cereal and marshmallows until the color blends through the entire batch of treats.*
Pour into a well-buttered 9″X13″ pan.
Quickly, while the marshmallow is still warm and sticky, add sets of 3 different sized heart sprinkles on top of the treats in 7 rows of 4.
This will make 28 relatively square treats. If you want larger treats then add fewer sets of the hearts.
Allow to cool for at least 30 minutes, preferably an hour or so, before cutting into rectangles or squares.