Cockles are sometimes called blood clams because the liquid that the raw (or lightly blanched) bivalves release when opened is dark red. They look similar to clams, except that cockle shells have deep, narrow ridges that radiate out from the hinge.
Cockles aren't that easy to find in fish markets. If you can't get them, substitute small clams.
To clean cockles, start by giving them a good rinse in a colander. Keep them in the colander and prepare two bowls of water: one for washing the cockles, and a second one of salted water (75g [¼ cup and 2tbsp] of sea salt dissolved in 1½ litres [1½ quart] of water) to put the scrubbed cockles into so that they can purge themselves of any mud in the shells.
Use a stiff toothbrush to clean the cockles, occasionally dipping them and the toothbrush into the first bowl of water to rinse away mud (change the water in this bowl whenever it becomes dirty). When each cockle is cleaned, place it into the bowl of salted water. Discard any cockles with broken shells.
Once all the cockles have been cleaned, leave them in the salted water for several hours so they can spit out the mud in the shells. If the water becomes dirty, drain the cockles, rinse out the bowl, then add fresh water and salt in the same amounts as before.
When cooked briefly, cockles often remain tightly closed, unlike clams and mussels, which open when heated. These cockles are blanched for only 30-60 seconds, so they are still basically raw. They should not be eaten by the very young or the very old, or by anyone else with a compromised immune system. And as with all seafood, buy your cockles from a reputable supplier.
Don't be surprised at the use of fish sauce in this recipe - it's a common ingredient in Chiu Chow cuisine. This sauce is also delicious poured over raw shrimp: cut each shrimp down its back and remove the vein, but keep the shells on. Marinate the shrimp for a couple of hours in the fridge.
Clean the cockles, then let them in salted water for about three hours, changing the water as needed.
In a medium-sized bowl, mix the fish sauce with the sake, then add the sugar and stir until dissolved. Chop the garlic and cut the chillies into thin rounds, squeezing out as many seeds as possible. Add the garlic and chillies to the fish sauce mixture.
Bring a large pot of salted water to the boil. Place the cleaned cockles in a colander, then carefully pour them into the boiling water. Bring to the boil and cook for 30-60 seconds, then drain in a colander. Shake the colander to rid the cockles of as much water as possible, then put them – still hot – into the bowl containing the fish sauce and sake mixture. Mix thoroughly, then leave at room temperature until the cockles are barely warm (about 15 minutes).
Roughly chop the coriander leaves and mix them in, then transfer all the ingredients to a flat container with a lid. Refrigerate for about four hours, mixing the ingredients often so they’re evenly marinated. Serve the cockles cold, and let your guests pry the shells open (it’s not easy).