Spray a 9 x 9 pan with non-stick spray and line it with parchment paper. Spray again.

In a microwave safe bowl, place butter, salt, and marshmallows. Microwave in 30 second increments, stirring in between until completely melted and smooth.

Add in rice krispie cereal and mix well. Pour into prepared pan and try to even out as much as possible. Sprinkle in M&Ms.

Chill in fridge for at least 30 minutes before cutting into squares.