Peel the nagaimo and grate it finely into a bowl. Stir in the water right away to keep it from browning.

Fill a 30 cm / 12-inch steamer pot with a few inches of water and bring it to a rolling boil. Line an 18 cm / 7-inch round cake pan with parchment and lightly grease the sides.

In a large bowl, whisk the egg whites with the salt until foamy, about 1 minute. Don't take them to stiff peaks.

Add the sugar in two additions, whisking after each, until the mixture is pale and slightly thickened, about 3 minutes.

Fold the grated nagaimo into the egg mixture until evenly combined.

Sift the flour and baking powder over the batter in two additions, folding gently each time until no dry streaks remain. The batter should be thick but still pourable.

Pour the batter into the prepared cake pan and tap it on the counter twice to release large air bubbles.

Cover the pan loosely with foil to keep condensation from dripping onto the top. Place it in the steamer basket.

Steam over a rolling boil for 35 to 40 minutes, until a skewer inserted in the center comes out clean and the top springs back when pressed.

Remove the pan carefully and let it cool in the pan for 10 minutes. Turn the cake out onto a rack and cool completely, about 15 minutes, before slicing thin.