In a heavy medium saucepan combine the sugar, cornstarch, and salt. Stir in the milk and the peanut butter pieces. Cook and stir over medium heat until peanut butter pieces are almost melted and mixture is thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. (All peanut butter pieces may not be melted.)
Gradually stir about half of the milk mixture into the beaten egg yolk. Return egg mixture to saucepan. Bring to a gentle boil. Reduce heat. Cook and stir for 2 minutes more using a wire whisk to help blend peanut butter pieces. Remove from heat. Stir in the vanilla. Pour mixture into a medium bowl; cover surface with plastic wrap. Chill thoroughly. (Small flecks of chips may still remain.) In a medium chilled mixing bowl beat whipping cream until soft peaks form. Fold whipped cream into the chilled peanut butter mixture. Spoon into individual dessert dishes and chill. Serve topped with chopped peanut butter and chocolate candies, if desired. Makes 6 servings.