Fill a 3-quart saucepan 3/4 full with water. Squeeze in the juice of 1/2 lemon and add salt, peppercorns and parsley. Bring water to a full rolling boil.
Remove pan from heat and add shrimp. Cover the pan and poach the shrimp for 4 to 5 minutes.
While you are waiting, prepare and ice bath. When the shrimp are pink and opaque, remove to the ice bath and let chill.
After the shrimp have been chilled through, remove to a paper towel to drain completely.
Either serve immediately with cocktail sauce or refrigerate until ready to serve, ideally within 12 hours.
Notes
You can use other size shrimp, but you may need to increase or decrease the poaching time accordingly.