Rinse the Rice: Place the short-grain rice in a large bowl or fine mesh strainer. Rinse under cold running water, gently swishing the grains with your hands. Drain and repeat 3 to 4 times until the water runs almost completely clear. Shake off excess water.

Cook: Transfer the rinsed grains to your rice cooker pot. Add the water (and the piece of kombu, if using). Close the lid and start the machine on the standard white rice setting or the specific sushi rice setting if your machine has one.

Make the Seasoning: While the machine is running, combine the rice vinegar, granulated sugar, and salt in a small saucepan over medium-low heat. Stir just until the sugar and salt have completely dissolved. Do not let it boil. Remove from heat and let it cool.

Prepare for Mixing: Once the machine chimes, let the pot sit closed for 10 minutes to steam. Remove the kombu. Transfer the hot grains to a large wooden bowl (hangiri) or a wide glass dish. Do not scrape the bottom of the pot if any grains are stuck or browned.

Fold and Fan: Pour the cooled vinegar seasoning evenly over the hot grains. Using a wooden paddle, gently "slice" through the grains at an angle to fold the seasoning in. Avoid mashing the grains. If possible, fan the mixture with a piece of cardboard or a small fan while folding to give it a beautiful, glossy shine.

Cool and Use: Cover the bowl with a damp towel to keep the mixture from drying out. Let it cool to room temperature before using it to make rolls or bowls.