-cut with a heavy knife & -washed Combine the rice vinegar, sugar, salt and mirin in a 1 to 1 1/2 qt enameled or stainless steel saucepan. Bring to a boil uncovered and stir in the soy sauce. Cool to room temperature. Combine 2 1/2 cups of cold water and the rice in a 1 1/2 to 2 qt stainless steel or enameled saucepan and let soak for 30 minutes. Then add the square of kombu and bring to a boil over high heat. Cover the pan, reduce the heat to moderate, and cook for about 10 minutes, or until the rice has absorbed all of the water. Reduce the heat to its lowest point and simmer another 5 minutes. Let the rice rest off the heat for an additional 5 minutes before removing the cover and discarding the kombu. Transfer the hot rice to a large non-metallic platter or tray. Pour on the vinegar dressing and mix thoroughly with a fork. The rice is ready to use when it has cooled to room temperature. Or it may be covered and left at room temperature as long as 5 hours before serving.