In a mediumsize saucepan, combine butter, evaporated milk, granulated sugar and salt. Bring to a boil over medium heat, stirring constantly. Boil for 45 minutes.

Butter an 8 x 8 baking dish (but don't burn the fudge mixture while you're buttering the dish). Remove the pot from the heat and stir in marshmallows, chocolate morsels, vanilla and nuts (if desired). Mix vigorously for 1 minute or until marshmallows are melted. Pour into buttered baking dish and chill until firm.