Nerikuri (Sweet Potato Glutinous Rice Cake)

Rinse 1.5kg glutinous rice three times, soak it in 3L water under refrigeration for 10 hours, and drain it for 30 minutes. Cut 3kg peeled sweet potato flesh into 4cm pieces, cover the pieces with cold water, hold them there for 10 minutes, and drain. Line both tiers of a steamer with damp cloth and keep the ingredients separate. Steam the rice for 35 minutes and the potato for 25 minutes over strong steam. Both should crush easily between the fingers. Pound the hot rice alone for 8 to 10 minutes, stopping only when individual grains are no longer visible. Add the hot sweet potato in three portions and pound for 3 minutes after each addition. Combine 1kg white sugar with 1 tablespoon salt and add this mixture in four portions. Work for 2 minutes after every addition so each one disappears before the next goes in. The yellow dough is ready when it contains no white sugar streaks, intact rice grains, or separate potato lumps. If either steamed component cools before mixing, return it to the steamer in its cloth for 3 minutes so both enter the pounding bowl hot. Spread 200g roasted soybean flour 5mm deep across the bottom of a large tray. Press the hot dough over it to an even 4cm thickness, cover the top with the remaining 100g kinako, and leave the tray at room temperature for 3 hours. Cut the set slab into 40 pieces. The flour above and below keeps the sticky surface manageable, while a clean interior confirms that the potato and rice were fully incorporated.