1. In heavy saucepan, heat sugar and water on medium-high until sugar dissolves and turns amber in color, this will take about 15 minutes.
2. Heat 1 cup of whipping cream. Reserve the rest for whipping later.
3. Remove saucepan from heat. Stir in warm cream until a smooth caramel forms. Stir in chocolate, salt and butter. Stir in vanilla extract and the walnuts, keep some on the side for sprinkling later.
4. Pour warm chocolate filling into pie shell. Cool. Place in fridge for at least 3-4 hours to set.
5. When ready to serve. Whip remaining cream, spread on top and sprinkle with remaining chopped walnuts.