Wash the white rice thoroughly until the water runs clear. Steam the washed rice according to your rice cooker's instructions, or boil with 1.5 cups of water until fully cooked.

Spread the cooked rice on a tray and let it cool down to approximately 60°C (140°F). It's crucial not to let it get too cold or too hot.

In a large bowl, combine the cooled rice with the rice koji. Use your hands to thoroughly break up any clumps and mix the koji evenly with the rice.

Pour in the 2 cups of warm water (around 60°C/140°F) and mix well. The mixture should be moist but not soupy.

Transfer the mixture to a clean, sanitized container suitable for fermentation, such as a large jar, a slow cooker liner, or an Instant Pot insert.

Ferment the mixture at a constant temperature of 55-60°C (130-140°F) for 8-12 hours. Stir the mixture every 2-3 hours to ensure even fermentation and temperature distribution.

Taste the amazake periodically. Once it reaches your desired level of sweetness, the fermentation process is complete. The longer it ferments, the sweeter it generally becomes.

Once done, immediately cool down the amazake to stop fermentation. Transfer it to an airtight container and store in the refrigerator. It can be stored for up to one week, or frozen for longer preservation.