Place a sieve over a bowl, add rice, and rinse it with water. Repeat this process several times, changing the water each time.
Soak the rinsed rice in water in a pot for at least 30 minutes.
Put water (as specified in the recipe) in a pot and bring it to a boil. Once boiling, reduce the heat to low, add bonito flakes, and let it simmer for 3 minutes.
Turn off the heat and strain the mixture through a sieve lined with paper towels or a cloth (such as cheesecloth). Alternatively, if you don't mind having some fine bonito flakes remaining, you can simply use a fine-mesh strainer. Bonito dashi is now ready.
Cut bamboo shoots into bite-sized pieces along the fibers for the upper part, about ⅕-inch (5mm) thick. For the bottom to middle part, slice them into bite-sized pieces against the direction of the fibers, about ⅕-inch (5mm) thick. Finely chop the aburaage.
Discard the water in which the rice was soaked from the pot. Combine the drained rice with the dashi (⅔ cup/160 ml per 2 servings) and seasonings (sake, mirin, salt, and soy sauce). Then, evenly distribute the bamboo shoots and aburaage on top.
Cover the pot and bring it to a boil. Once boiling, reduce the heat to low and continue cooking for 10 minutes, or slightly longer for larger quantities (11 minutes for 4 servings, 12 minutes for 6 servings).
Turn off the heat and let it steam for 10 minutes with the lid still on.
Serve in each bowl and top with sansho leaves for color (optional).