Heat the butter in a large pot over low heat until melted.
Add 6 cups of marshmallows to the pot and melt, stirring constantly.
Once the marshmallows are melted, remove the pot from the heat and stir in the Rice Krispies and remaining 2 cups of marshmallows. Stir to coat.
Add in all but a handful of the m&ms and the sprinkles and stir to combine.
Spread the mixture into a greased 9x13 baking dish. Use a piece of greased wax paper to press lightly into the pan.
Sprinkle with the remaining m&m’s and press into the pan again.
Let sit for 1 hour before cutting and serving.
We use plain m&m's in this recipe, but any variety should work fine. When adding in the extra marshmallows, you'll want to stir them in with the Rice Krispies to soften, but don't try to melt them fully. They add little bits of chewy marshmallow throughout the bars that everyone loves.