Line an 8 or 9-inch square pan with foil, extending foil over edges of pan; butter foil.
Stir together sugar, cocoa and salt in a heavy 4-quart saucepan; stir in milk.
Cook over medium heat, stirring constantly, until mixture comes to full rolling boil.
Boil, without stirring, until mixture reaches 234 degrees on a candy thermometer, or until a small amount of mixture dropped into very cold water forms a soft ball;
remove from heat.
Add butter, vanilla, and marshmallow cream; do not stir.
Cool to 110 degrees.
Beat mixture for 8 minutes, stir in chopped nuts.
Quickly spread into prepared pan and cool completely.