Combine butter, evaporated milk, sugar and salt in sauce pan over medium heat, stirring occasionally. Bring to full boil. Cook 4 to 5 minutes, stirring constantly. Remove from heat. Stir in marshmallows, chocolate pieces, vanilla and nuts. Stir vigorously for 1 minute (until marshmallows melt and blend). Pour into 8 inch square buttered pan. Cool. Cut into squares.