Sieve the boiled yolks through a medium mesh strainer. In a small nonstick pan, combine the yolks, sugar, and salt, and cook over low heat until yolks become dry, 6 to 7 minutes. Stir often, taking care not to let the mixture burn. Set aside.
In another small nonstick pan over low heat, cook the katsuobushi (first crush it by hand) and soy sauce until dry, abou