In a heavy 3 qt saucepan, heat sugar and water on medium until sugar dissolves and turns amber colored, about 15 minutes.
Meanwhile, in microwave safe 1 cup measuring cup, heat 3/4 cream on high for 45 seconds. Place remaining 1/2 cup cream in refrigerator to keep cold for whipping later.
Remove saucepan from heat. Stir in warm cream until a smooth caramel forms, it will stiffen when cream is added. Stir in chocolate and butter until completely melted. Stir in vanilla and 1 1/2 cups walnuts. Reserve remaining walnuts to sprinkle on top of pie later.
Pour warm chocolate filling into pie shell. Cool on wire rack for 1 hour, then cover and refrigerate for 3 hours or until set.
When ready to serve, in a medium bowl, with mixer on medium speed, beat remaining 1/2 cup cream until soft peaks form. With metal spatula, spread whipped cream over top of pie. Sprinkle with reserved walnuts.