Jaga-imo Manju (Shrimp-Stuffed Potato Buns)

Put potatoes into a medium pot of salted water and boil over medium heat until soft, 20–25 minutes. Meanwhile, finely grate ginger into a small bowl. Squeeze juice from half the grated ginger into another small bowl, discarding spent ginger, and set juice aside. Bring sake, 1 tbsp. of the rice wine, and 2 tsp. of the soy sauce to a boil in a small pot over high heat. Add shrimp and cook, stirring, until just cooked through, about 1 minute. Drain shrimp, discarding cooking liquid, and set aside.

Drain potatoes, return to pot, and cook over medium heat, shaking pot over heat, until moisture evaporates and potatoes break apart, 1–2 minutes. Pass potatoes through a potato ricer into a bowl, then press through a sieve with a wooden spoon into a large bowl. Add butter, a pinch of salt, two-thirds of the egg white, and 2 ½ tsp. of the cornstarch to potatoes and stir until dough is smooth. Quarter dough, shape into 4 balls, then flatten into ¾"-thick disks. Put one-quarter of the shrimp into center of each disk, gather dough up and around filling, and press to seal, pinching dough together if it tears. Shape stuffed dough into smooth balls and brush with the remaining one-third egg white. Put 2 ½ tsp. of the cornstarch into a shallow dish in an even layer. Gently roll dough balls in cornstarch to coat lightly. Transfer dough balls to 4 small heatproof bowls, cover with plastic wrap, and set aside.

Bring ⅓ cup of the dashi, 1 tbsp. of the soy sauce, and 1 tbsp. of the rice wine to a boil in a small pot over high heat; set aside to cool. Meanwhile, blanch spinach in a small pot of salted boiling water over high heat; then drain. Add spinach to dashi mixture and set aside to marinate for 10 minutes. Drain spinach, discarding marinade, and set aside.

Uncover dough balls, transfer bowls to a bamboo steamer basket, cover with steamer lid, and steam over a wok of boiling water over medium-high heat until buns are cooked through, about 15 minutes.

Meanwhile, dissolve the remaining 3 tbsp. cornstarch in 3 tbsp. cold water in a small bowl and set aside. Put the remaining 1 ⅔ cups dashi, 2 tbsp. soy sauce, 1 tbsp. rice wine, reserved ginger juice, and pinch of salt into a small saucepan and bring to a simmer over medium heat. Gradually add dissolved cornstarch, stirring constantly until sauce thickens, 2-3 minutes. Remove bowls from steamer, spoon some of the sauce around each potato bun, and garnish each with reserved spinach and some of the remaining grated ginger.