1. In a large pot over medium-low heat, melt butter. Add marshmallows and stir until completely smooth. Remove from heat. Add crispy rice cereal. Stir with rubber spatula coated in cooking spray until fully combined. 
2. Evenly press mixture into a 9"x 9"pan lined with parchment paper. Cool for 40 minutes.
3. Cut into evenly sized rectangles.

For Monster Making:

1. Melt candy wafers according to package instructions.
2. Dip the Krispie treats halfway into the melted candy. Use a butter knife to make sure that monsters candy coating is even and covers all desired areas. Set on parchment paper.
3. While coating is still warm, gently press the candy eyes into place and use the sprinkles to add "hair" to your monster. Allow treats to set for 45 minutes prior to serving.