Heat a large pot over medium heat. Add the butter and melt.

When the butter has melted, stir in the marshmallows. Continue stirring, until marshmallows have just melted. Remove from the heat. 

Stir in the cereal until mostly coated in marshmallows.

Add the sprinkles and most of the m&m candies. Reserve a handful of m&m's for sprinkling over the tops of the bars. Stir the mixture together.

Press the mixture into a greased 9x13 baking dish. Press the remaining m&m's onto the top of the bars.

Let cool 10 minutes before cutting and serving.