Giant Frankenstein Rice Krispie Treat Cake

Prepare the pan

1. Grease a 9x13-inch baking dish with butter, then line it with parchment paper, leaving a generous overhang on the two long sides. The parchment lets you lift the whole treat out in one piece and place it on a board like a cake. Measure the cereal into a large mixing bowl and set aside 1 cup of the mini marshmallows.

Melt the marshmallows

1. Melt the butter in a large saucepan over low heat. Add all the mini marshmallows except the reserved cup and stir constantly with a spatula until completely melted, smooth and glossy, about 5 to 6 minutes. Keep the heat low throughout. It may feel slow, but gentle heat is what keeps the finished treat soft and chewy.

Add the color and flavor

1. Remove the saucepan from the heat. Stir in the salt, vanilla and green gel food coloring until the color is even with no streaks remaining. The mixture should be bright green and glossy. If you would like a deeper shade, add a little more coloring now, before the cereal goes in.

Fold in the cereal

1. Pour the green mixture over the cereal and add the reserved cup of marshmallows. Fold gently with a greased spatula until every piece of cereal is coated, about 1 to 2 minutes. The mixture will be thick and sticky, and the reserved marshmallows will only partly melt. This is normal and gives the treat soft, gooey pockets.

Press and set

1. Transfer the mixture to the prepared dish. Using a greased spatula or a lightly greased piece of parchment, press it into an even layer, reaching into the corners. Press just firmly enough to hold it together, as packing it tightly will make it dense. Leave it at room temperature for about 40 minutes, until firm to the touch.

Move to a serving board

1. Lift the treat out of the dish using the parchment overhang and slide it onto a large serving board or platter, peeling the parchment away. Turn it so one of the short 9-inch sides is at the top, as this becomes the forehead and hairline.

Melt the chocolate

1. Place the chocolate chips and coconut oil in a small microwave-safe bowl. Heat in 30-second intervals, stirring well after each one, until smooth and glossy, about 1 to 1 1/2 minutes in total. If a few small lumps remain, keep stirring rather than heating again. Spoon about 2 tablespoons of the chocolate into a small zip-top bag and set it aside for the brows.

Add the hair

1. Pour the remaining chocolate across the top quarter of the treat. Spread it with the back of a spoon to cover the top edge, then pull the chocolate downward in short strokes to create a jagged hairline. Scatter the chocolate sprinkles over the chocolate while it is still wet so they stick.

Make the eyes

1. Cut the large marshmallow in half crosswise with kitchen scissors. Press each half, cut side down, onto the green face just below the hairline, leaving a small gap between them. The cut side is sticky and will hold without any extra help. Dab a tiny amount of chocolate from the bag onto each marshmallow and press on a candy-coated chocolate for the pupils.

Pipe the brows and mouth

1. Snip a very small corner from the zip-top bag and pipe a thick, straight brow above each eye. Using the black decorating icing, pipe a wide line for the mouth, then add short crossing lines to make stitches. You can also pipe a small stitched scar on the forehead if you like.

Add the neck bolts

1. Gently push one pretzel rod half into each long side of the treat, near the bottom, so they stick out like neck bolts. Twist them slightly as you push, and they will slide in without breaking the treat.

Chill and serve

1. Place the decorated treat in the refrigerator for about 15 minutes, until the chocolate is firm and set. Let it sit at room temperature for 5 minutes before slicing so the base softens slightly. Cut into 16 pieces with a large knife lightly greased with butter, wiping it between cuts for clean edges.