Place the butter into a large saucepan and melt over low heat.
Add the marshmallows and stir until completely melted.
Slowly add the Rice Krispies to the melted marshmallow and butter mixture and stir together to coat well.
Spray a non-stick 8" square pan with cooking spray and transfer the cereal and marshmallow mixture to the pan.
Use some wax paper or the back of a spoon or spatula dipped in water to evenly press the mixture down.
Press M&M's onto the top of the cereal mix and place into the fridge to cool for 1-2 hours.
Cut into 2-inch squares and enjoy! Best served on the same day.
Video
Notes
You can melt the butter and marshmallow mixture in the microwave if you prefer. Melt the butter for 30 seconds and stir, then add the marshmallows and microwave for a further 30-60 seconds until melted. Then follow directions as above.