Measure out all of your ingredients in advance - this recipe comes together quickly.
Line a 9x13 pan with parchment paper, letting some of the paper hang over the sides of the pan for easy removal later.
Start by melting the butter in a large pot. Once melted, add the marshmallows, and stir constantly until the marshmallows are almost completely melted. Add the salt, stir, and then add the Rice Krispies and stir to combine.
Add the M&M's to the mixture and stir to combine.
Pour the mixture out into your prepared pan, and press it out into an even layer. Cool for 30-60 minutes, and cut into squares.
I like to not fully melt the marshmallows, so that you end up with some pockets of marshmallow in the treats that are extra delicious.