Whisk sugar, cornstarch, and salt together in large saucepan. Whisk in cream until combined. Add butter and cook over medium heat, whisking constantly, until mixture is thick and large bubbles appear at surface, 6 to 8 minutes. Continue to whisk 20 seconds longer. Off heat, whisk in vanilla. Pour custard into pie shell and smooth top with rubber spatula. Let pie cool completely, about 2 hours. Refrigerate pie until cold, about 2 hours. Grate nutmeg over pie and serve.