Combine butter, evaporated milk, sugar and salt in medium, heavy saucepan. Bring to a boil over medium heat, stirring constantly. Boil for 4 to 5 minutes, stirring constantly. Remove from heat.
Stir in marshmallows, morsels, nuts and vanilla. Stir vigorously for 1 minute or until marshmallows are melted. Pour into foil-lined 8-inch square baking pan; chill until firm. Cut into 1 1/2 inch squares.