Line a 9x13-inch pan with parchment paper and lightly spray with cooking spray.
In a large pot, melt the butter over medium heat, then add the mini marshmallows. Stir and continue stirring until the marshmallows are completely melted. Add the vanilla extract and salt and stir to combine.
1 stick butter, 16 ounces mini marshmallows, ½ teaspoon vanilla extract, ¼ teaspoon kosher salt
Remove from the heat and add the cereal. Use a wooden spoon to stir until the marshmallow is evenly distributed and all of the pieces of cereal are coated. Make sure to get any remaining marshmallow from the bottom of the pot. If your pot is too small, you can also transfer the mixture to a very large bowl to combine everything more easily.
8 cups rice krispies cereal
Pour in the mini M&Ms and stir until they're just evenly combined, but try not to overmix as this will lead to the colors spreading (see note).
1 cup mini M&Ms
Pour the mixture into the prepared pan, and use a greased spatula to gently spread it evenly. Top with additional mini M&Ms if desired.
Let sit at room temperature for roughly 1 hour, then remove from the pan and cut into 12 squares. Enjoy immediately for the best texture!
Notes
To ensure the M&Ms don't melt too much, I like to let the mixture cool for a few minutes before pouring them in so that the colors don't run.