Line a 9x13 with parchment paper. Lightly grease the parchment. I always use a very light mist of nonstick spray. Set aside.
Melt the butter over low heat in a large pot or Dutch oven. Once melted, add the 10-ounce bag of marshmallows. Stir the mixture until the marshmallows are just melted. Stir in the salt and vanilla to combine.
Remove the pan from the heat, add the Rice Krispies, and mix until coated with the marshmallow mixture.
Immediately stir in an additional one cup of marshmallows and cup of M&M's.
Turn the mixture out into the prepared pan and, using a wooden spoon or spatula that has been coated in non-stick cooking spray, lightly press the mixture into an even layer. Do not pack it down with force.
Lift the treats as a whole out of the pan using the edges of the parchment paper. Use the cookie cutters to cut out shapes.
Let treats rest for 30 minutes. Cover and store leftover treats at room temperature for up to 3 days. To keep, place in an airtight container, with sheets of parchment or wax paper between layers.