Place butter in a large, heavy-bottomed pan over medium heat. Melt butter, stirring frequently. Butter will bubble, then turn golden brown, and it will smell nutty. Don't be alarmed if there are small bits of dark brown. Remove from heat.
12 Tablespoons Butter
Add sweetened condensed milk, vanilla, and Kosher salt. Stir until combined.
¾ cup Sweetened Condensed Milk, 1 ½ teaspoons Vanilla Extract, ½ teaspoon Kosher Salt
Add marshmallows and return to medium-low heat.
10 ounces Mini Marshmallows
Stir vigorously until the marshmallows have melted. Remove from heat.
Add Rice Krispies cereal and fold cereal in until well combined. Allow to cool for 2 minutes to prevent chocolate from melting.
9 cups Rice Krispies Cereal
Gently fold in half of the M&Ms.
½ cup M&M Candies
Pour Rice Krispies into a greased 9x13 pan. Dampen your hands in cold water and gently press down the cereal mixture.