Fill a medium bowl with cold water and add 1 tablespoon of salt. Add the clams and let stand for 1 hour. Drain the clams and rinse them well. Discard any open or broken shells.
In a large, deep skillet, combine the sake with the water and bring to a boil. Add the Manila clams, cover the skillet tightly and cook, shaking the pan occasionally, until most of the clams have opened, about 4 minutes.
Spoon the clams and broth into 4 medium bowls. Top with the butter, garnish with the scallions and togarashi and serve at once.