In a large bowl, combine butter and marshmallows. Heat it in the microwave for 1 minute. Mix well and cook for another 45 seconds, until completely melted.
Add the Rice Krispies cereal to the marshmallow mixture. Mix well.
Cover the bottom and the sides of a large baking pan (13x9) with parchment paper and lightly grease it.
Spread the rice krispie mixture in the pan, evenly. Press lightly and cool in the fridge for about 15-20 min.
Using an egg shaped cookie cutter, cut out the rice krispie eggs. (I suggest lightly greasing your hands while handling the rice krispie treats).
Using a small plastic spatula, cover the sides and the top of the egg with a thin layer of marshmallow fluff.
Quickly, add the M&Ms to the sides and the top of the eggs. Press lightly.