Wash the shijimi clams thoroughly by rubbing the shells together under plenty of water, then drain.

In a frying pan, add the shijimi clams and water, then heat over medium heat until the clams open.

Remove the clams, skim off any foam, and add the spaghetti (broken in half). Cook for 1 minute longer than the package instructions, stirring occasionally.

Dissolve the miso into the pan, then add butter and soy sauce. Return the clams to the pan and mix well.