Combine butter, evaporated milk, sugar and salt in medium, heavy saucepan. Bring to a boil over medium heat, stirring constantly. Boil for 4 1/2 to 5 minutes, stirring constantly. Remove from heat. Stir in marshmallows, morsels, nuts and maple flavoring. Stir vigorously for 1 minute or until marshmallows are melted. Pour into foil-lined 13x9-inch baking pan. Place walnut halves in rows, spacing about l/2-inch apart on top of fudge. Press into fudge. Chill until firm. Cut into squares with 1 walnut half per square. Walt Mm