Place a large bowl on crushed ice. Pour cider and sour cream into bowl and combine. Add chicken, cucumber, egg whites and yolks. Add dill and scallion, salt and white pepper. Finally, add tarragon vinegar to taste. Chill for 2 hours before serving.
For garnish: Fry bacon in butter for 1 minute. Add bread and cook until crisp and brown. Toss in parsley to coat.