Line a brownie-sized baking pan with wax paper or foil, covering sides of pan. Spray with non-stick spray or grease foil with butter. Combine butter, sugar, and evaporated milk (do not use sweetened condensed milk) in a saucepan stirring over medium heat until butter is melted and sugar is dissolved. While constantly stirring, turn to high heat and allow to reach a rolling boil. Once mixture is boiling, stop stirring and allow to boil for an additional 5 minutes, then remove pan from heat.

Stir in marshmallows; when melted, add chocolate and continue to stir until all is melted and well combined. Stir in chopped nuts. Immediately spread mixture in the lined baking pan. Allow to cool before cutting.