In a large saucepan over low heat, melt the butter. Once melted, add the mini marshmallows and stir constantly until completely melted and smooth.
Remove the saucepan from the heat and stir in a few drops of green food coloring. Mix until the marshmallow mixture is evenly tinted a bright Grinch green.
Add the Rice Krispies cereal to the marshmallow mixture and stir gently until the cereal is evenly coated.
Transfer the cereal mixture into a greased 9x13-inch baking pan. Press it down firmly to form an even layer. Let the treats cool for a few minutes, then top with red candy hearts or M&Ms to resemble the Grinch’s heart.