Drain the firm tofu and wrap it in a clean kitchen towel to press out excess moisture. Let it sit for about 15 minutes, then cut into 3cm cubes.

In a bowl, mix equal parts potato starch and all-purpose flour. Carefully coat each tofu cube in this mixture, ensuring all sides are evenly covered.

Blend the remaining potato starch with about 1 teaspoon of water to form small clumps known as 'hailstones.'

Heat oil to 170°C (340°F) over medium heat. Monitor the temperature closely.

Roll the coated tofu cubes in the hailstones before frying.

Fry the tofu cubes for about 3 minutes, turning halfway through until light golden brown.

In a saucepan, combine dashi stock, soy sauce, mirin, sake, and salt. Bring to a gentle boil, then thicken with a potato starch slurry.

Serve the fried tofu in a bowl, drizzled with warm umami sauce and garnished as desired.