Heat a large pot over medium low heat on the stove. Add the butter and melt.
Add the marshmallows and stir occasionally until melted. Do not heat too high as it will cause the marshmallow to brown.
Remove from the heat.Add the cocoa cereal and stir to combine.
Pour the mixture into a baking sheet that was first lined with parchment paper, and then sprayed with non-stick spray.Spread the mixture into an even layer.
Let sit until cool, at least 30 minutes.
Remove the treats from the dish by lifting the parchment paper.
Cut the treats into 12 pieces by first cutting in half short ways, and then cutting each half in half again short way, giving you 4 columns.
Then cut all the columns into three even rows.
Trim any pieces as needed to get your spell book shape.
Pipe the design on each piece by doing the following:1) A squiggle line down the left side2) A circle in the middle right, with a dollop in the middle which a candy eye is placed on3) Swirled designed in both right corners4) If desired, the stitching marks across the book
Repeat with all remaining pieces.
Let dry until frosting is set and store in an air tight container for up to 5 days.
You may need more marshmallow depending on which type of cocoa cereal you use, as some have more edges than others and need more "glue".If more marshmallow is needed, simply melt quickly on the stove or in the microwave and add to combine before pressing in the pan.