Line an 8-inch square baking pan with parchment paper and set it aside.

Melt the butter in a large saucepan over medium heat.

Add the mini marshmallows to the melted butter and stir until fully melted and smooth.

Remove the pan from heat and stir in the green food coloring until the mixture is evenly colored.

Add the crispy rice cereal to the marshmallow mixture and stir quickly until all the cereal is coated.

Transfer the mixture to the prepared pan and press it down firmly with a spatula or lightly greased hands to create an even layer.

While the mixture is still warm, press the candy eyes into the top in a scattered pattern, spacing them to create monster faces.

Allow the squares to cool at room temperature for 30 minutes before cutting into 4 portions.