Step 1 Rinse the clams well. Soak in the salt water overnight.
Step 2 Drain the clams in a strainer.
Step 3Pour the dashi into a medium-sized saucepan. Add the clams and cook over low heat until one of the clams opens, then increase the heat to high and cook until all the clams are open. Remove from heat. Skim the surface of any foam and discard.
Step 4Remove the clams from the saucepan and pour the broth through a paper towel-lined strainer, then return it to the saucepan. Remove half the clams from their shells, leaving the other half attached. Discard the empty shells.
Step 5Thin the miso with one-half cup of the broth. Taste the broth, and add enough of the thinned miso to lend flavor without making the broth too salty (the amount needed will vary depending on the salinity of the dashi and the saltiness of the miso; you might not use all the miso).
Step 6Stir the sake into the broth, then add the clams (both in and out of the shell). Bring to a simmer.
Step 7Divide the broth and clams between 4 bowls. Serve, garnished with chives.