Rinse the white rice thoroughly until the water runs clear. Cook the rinsed rice with 3 cups of water in a rice cooker or pot. The rice should be slightly softer than usual. Once cooked, spread the hot rice onto a clean tray or plate to cool down to about 60°C (140°F). If it's too hot, it will kill the koji enzymes. In a large, clean pot or a yogurt maker, combine the cooled rice, the rice koji, and 2 cups of water. Mix well to ensure the koji is evenly distributed throughout the rice. Cover the pot and maintain a temperature of 55-60°C (130-140°F) for 8-10 hours. Stir occasionally every 2-3 hours. After 8-10 hours, taste the amazake. It should be noticeably sweet. Once it reaches your desired sweetness, transfer it to an airtight container and refrigerate. It can be stored for about 1 week. Enjoy warm or chilled, plain or diluted with water, milk, or in smoothies.