Line a large baking sheet with parchment paper, and arrange the graham crackers, 4 across and 3 down.
Melt the butter, then pour over the marshmallows in a medium microwave safe bowl.
Heat the marshmallows in 30 second intervals, stirring between each heating session, until fully melted.
Grease a silicone spoon, and spread the melted marshmallows over the layer of graham crackers.
Add the chocolate chips to a microwave safe bowl, and heat in 30 second intervals, stirring between each heating session, until fully melted.
Spread the melted chocolate over the marshmallow layer.
Immediately top with M&Ms and nonpareils.
Chill for 1 hour, then cut into squares.