Coat the baking pan and a large mixing bowl with cooking spray.

Melt the butter in a large microwave-safe bowl in 30-second bursts. Add the mini marshmallows and microwave in 45-second bursts, stirring between each, until the marshmallows are fully melted and smooth.

Stir in the green gel food colouring until the mixture is an even bright green. Work quickly as it starts to firm.

Pour the puffed rice cereal into the green marshmallow mixture and stir until every piece is coated.

Press the mixture firmly and evenly into the prepared baking pan. Let it set at room temperature for 30 minutes.

Once set, cut into 24 equal rectangles.

Press two mini chocolate chips near the top of each bar as eyes. Use the black gel icing to pipe a straight-line mouth and a short row of hair lines at the top. Press a black sprinkle on each lower corner as a bolt.