Butter 8-inch square pan. In medium saucepan, combine butter, evaporated milk, sugar and salt. Bring to a boil over medium heat, stirring constantly. Boil 4 to 5 minutes, stirring constantly; remove from heat. Stir in marshmallows, morsels, vanilla and nuts. Stir vigorously for 1 minute or until marshmallows melt completely. Pour into pan. Cool and cut into squares. Hint: For thicker fudge, use a 7X5-inch loaf pan.