Prepare the kombu filling:

• In a bowl, soak the kombu in warm water for about 5-10 minutes, until it is soft enough to cut through.
• Drain, but save the water. Carefully cut the kombu into thin strips about ½ inch in length. These will be slimy, so be careful when you cut.
• Put the kombu back into the water, and add 2 tablespoons of rice vinegar. Allow it to soak for another hour or so. See notes
• Place the kombu and water-vinegar mixture in a saucepan. Mix in the soy sauce, sake, mirin, and sugar. Bring it up to a simmer and reduce the heat on the lowest setting. It should be barely bubbling.  
• Cook the kombu for about 20-30 minutes. Add a little bit of water if it starts drying out before it is ready. The kombu should be extremely soft, and there should not be any liquid left in the pan.
• When the kombu is done, it will be shiny and caramelized. Set aside.

Prepare Spicy Tofu:

• Cut a block of tofu in half, save the other half for something else. Press the tofu to release the excess water. About 30 minutes.
• Crumble the tofu with your hands until you get small pieces.
• Heat about a bit of oil in a large pan over medium heat. Place the tofu in the pan, along with the soy sauce and mirin. Mix to combine.
• Cook the tofu until it starts to brown. Remove from heat, and transfer it to a bowl. Allow it to cool. 
• Mix in the mayo and sriracha with the tofu. Set aside.

Prepare rice (Skip if you have a rice cooker):

• In a bowl, rinse the rice under cold water. Use your hands to swish the rice around to release the starches. Then drain the water. Repeat 3-4 times until the water runs clear. 
• In a pot, combine the rinsed rice and water. Cover and allow it to come to a boil over medium heat. 
• Once water is boiling, turn the heat to the lowest setting and continue to cook covered for 12 to 13 minutes, or until the water is completely absorbed. At 12-13 minute mark, take a quick peek and if you see there is any water left, close the lid and continue cooking for another minute or so
• Once the water is boiling, reduce the heat to the lowest setting. Continue to cook the rice, covered, for about 15 minutes. If there is water left at the 15-minute mark, continue to cook the rice for a minute or two.
• Turn off the heat and let the rice steam for about 10 minutes. Then transfer to a large bowl to cool down for a bit. Mix in the salt with the rice and let it cool down for a bit. You want the rice to still be pretty warm as we assemble.

Assembly:

* Before starting, cut the nori sheets in thirds (1/3).

If using a mold:

• Add about 1-2 teaspoons of salt to the rice and mix it together.
• With a damp cloth, wipe the entire inside of your onigiri mold until it’s completely damp. Also wet the removable top piece as well, on the side that will push into the rice.
• Fill your mold just below halfway with rice, and gently press it into all the crevices of the mold.
• Make a small indentation with a spoon or your fingers, in the center of your rice, deep enough to hold your filling. 
• Add in your filling of choice in the indent. Do not overfill.
• Fill in the rest of your mold with rice, until about the 3/4 point. Take the top piece and gently push until the top piece is fully inserted into the mold. 
• Remove the rice ball out of the mold and wrap a strip of nori onto the onigiri. Repeat until you run out of rice. Serve and enjoy!

Without mold:

• Fill a bowl with warm water and another with 3 tablespoons of salt.
• First, wet your hands so the rice won’t stick. Then dip your fingertips into the salt and rub to spread all around your palms. 
• Scoop out a handful of rice (about ½ cup) and create an indentation in the middle. Fill it with your filling of choice.
• Use your hands to shape and mold the rice over and around the filling. Press the rice around the filling to gently form the rice into a triangle. Don’t squeeze too hard. It should be firm enough so the onigiri doesn’t fall apart.
• Wrap a strip of nori onto the onigiri. Repeat until you run out of rice. Serve and enjoy!