Mix butter, chocolate chips, marshmallows, vanilla and nuts (if desired) in the large mixing bowl.
Bring the milk and sugar to a rolling boil in the heavy pan and cook for 10 minutes, stirring occasionally.
Pour sugar and milk mixture over other ingredients in mixing bowl.
Stir until all ingredients are well blended and marshmallows, butter and chocolate chips are dissolved.
Pour fudge into buttered foil or baking pans.
Let fudge stand 24 hours before eating, or cover and store in refrigerator.