In a large, microwave safe bowl, melt the marshmallows and butter on LOW for about 6 minutes, stirring halfway through, until smooth.
Add the chopped Snickers and Cereal and stir until thoroughly combined.
Scoop into a greased up 13x9 inch baking dish, and press down with a buttered spatula or fingers.
In a medium, microwave safe bowl, melt the chocolate and peanut butter on LOW for about 3 minutes, until no longer lumpy. Pour over the cereal bars and smooth out with a spoon or spatula.
Allow to set for several hours before cutting and eating. For faster setting, place in the fridge or freezer, covered, until hard.