1. Wash the rice thoroughly until the water runs clear. Cook the rice according to package directions, aiming for a slightly firmer texture than usual. Let it cool to approximately 60°C (140°F). It's crucial not to let it cool too much below this, as koji thrives in a specific temperature range.

2. In a large, clean bowl, break up the cooked rice gently to separate the grains. Add the crumbled rice koji and mix thoroughly with your hands, ensuring the koji is evenly distributed throughout the rice.

3. Gradually add 400ml of water, mixing well until the rice and koji mixture is consistently moist. The consistency should be like a thick porridge.

4. Transfer the mixture to a clean, sterilized container suitable for fermentation, such as a large jar, a rice cooker pot (if using its 'keep warm' function), or a slow cooker. Cover loosely to allow air circulation but prevent contaminants.

5. Maintain a constant temperature between 55-60°C (130-140°F) for 6-10 hours. Stir every 1-2 hours to ensure even fermentation. You'll notice a sweet aroma developing and the mixture becoming more liquid as the starch converts to sugar.

6. After 6-10 hours, taste the Amazake. It should be pleasantly sweet. If it's not sweet enough, continue fermenting for another hour or two, checking regularly. Once it reaches your desired sweetness, turn off the heat.

7. If you prefer a smoother texture, blend the Amazake using an immersion blender or a regular blender until smooth. For a chunkier texture, leave it as is.

8. Let the Amazake cool completely before transferring it to airtight containers. Store in the refrigerator and consume within one week. Serve chilled or gently warmed.