Clean the clams well in cold water; scrub any slimy shells. Discard any that are broken or won't close when tapped.
If using wild clams, purge them: place in salted water (3 Tbsp sea salt to 2 L water) for about 1 hour, undisturbed, to spit out sand.
Add the coconut milk, chicken stock, shallots, lemongrass, galangal, kaffir lime leaves, Thai chilies and sugar to a large pot. Cover and bring to a boil. Add the clams, cover and cook 2-3 minutes until they open. Remove from heat, add lime juice and cilantro, and stir. Adjust seasoning with fish sauce or salt.
Transfer to a serving bowl. Try steaming any unopened clams a little longer; discard any that still won't open.
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