Spray a 9x13 baking dish with non-stick spray.
Lay 12 Halloween Peeps along the bottom of the dish in any pattern you would like.
Pour 3 cups of Rice Krispies cereal over the marshmallow Peeps.
Add another layer of 12 Peeps over the cereal.
Pour the remaining 3 cups of cereal over the Peeps.
Slice the butter and place on top of the cereal.
Bake the treats at 350 degrees for 15 minutes to allow the marshmallows to melt.
Remove the dish from the oven and stir the contents until the cereal is covered with melted marshmallow mixture then press into the bottom of the pan.
Top the Rice Krispy treats with Halloween Sprinkles and allow the treats to cool for 1-2 hours before cutting into squares.