Rinse the short-grain white rice thoroughly until the water runs clear.

Cook the rice with 4 cups of water according to package directions, or until it's very soft and slightly mushy. Let the cooked rice cool down to approximately 140°F (60°C). This temperature is crucial for the koji enzymes.

In a large, clean bowl, combine the cooled cooked rice with the crumbled koji rice and 2 cups of water. Mix well with a clean spoon, ensuring the koji is evenly distributed throughout the rice.

Transfer the mixture to a clean slow cooker, an insulated rice cooker (using the 'keep warm' setting), or a yogurt maker. Maintain a consistent temperature between 130-140°F (55-60°C). This temperature range activates the koji enzymes without killing them.

Ferment for 6-8 hours, stirring the mixture gently every 1-2 hours. You will notice the rice grains breaking down and the liquid becoming increasingly sweet as the starches convert to sugars.

Once the desired sweetness is reached (taste it periodically), turn off the heat. The amazake is ready.

Serve warm as a comforting drink or chill it for a refreshing cold beverage. For an extra smooth texture, you can blend the amazake before serving. Store any leftovers in an airtight container in the refrigerator.