In large pot, melt butter and marshmallows together over medium heat. Once marshmallows have completely melted, take off the heat and stir in cinnamon, nutmeg, vanilla and rice krispie cereal. Stir to combine and pour out into prepared dish.
Press down with lightly greased fingers and set aside to cool.
Place chocolate chips and butter into a small bowl and microwave for 15 second increments, stirring each time. Once chocolate chips have almost melted completely, continue stirring until the chocolate is completely smooth.
Spread an even layer of melted chocolate over top rice krispie treats. Top with chopped Halloween candy. Krispie squares are ready to be eaten one chocolate has hardened. Cut and serve.