If the crab is in sticks, cut in half. If in chunks, roughly chop. Transfer to a small bowl and fold in the mayonnaise, if using, until well combined; set aside.
Combine the rice vinegar with 1 cup water in a small bowl. This will keep the rice from sticking to your hands.
Place a bamboo sushi mat with the slats running crosswise directly in front of you on a clean work surface. Lay 1 piece of the nori, shiny-side down, with a short side closest to you.
Lightly moisten your fingers with the vinegared water, and scoop up about 3/4 cup (a loose mound the size of a tennis ball) of the sushi rice with your hand and spread in a thin, even layer onto the nori, leaving a 1/2-inch strip uncovered at the edge closest to you. Flip over so the rice side is on the mat to make inside-out rolls.
Smear a dab of the wasabi crosswise across the center of the nori. Top with some of the crab, cucumber and avocado, without overstuffing the roll. Add a squirt of Sriracha, if using.
Lift up the uncovered strip of the nori and roll into a log over the rice and fillings with your fingertips as you roll. Wrap the roll in the mat, lining up one end of the roll against the edge of the mat. Firmly tug on the rounded mat over the roll as you pull on the edge of the mat to tighten the roll. Flip the roll over to line up the other end against the edge of the mat and repeat the tugging. Sprinkle sesame seeds over the rice.
Moisten the edge of a sharp knife, slice the roll in half crosswise. Line up the two halves. Wipe the knife with a damp towel and cut the 2 rolls crosswise into half again. Repeat with the remaining ingredients. Serve the rolls with wasabi and soy sauce
Place the rice in a fine-mesh sieve and rinse under cold running water, massaging the rice, until the water runs clear. Drain well.
Transfer the rice to a rice cooker, add the water and cook according to the manufacturer's directions. (Alternatively, combine the rice and water in a 2-quart heavy saucepan with a tight-fitting lid. Cover, and bring to a boil over high heat, then reduce the heat to low and cook for 20 minutes. Remove from the heat and let rest until all the liquid is absorbed and the rice is shiny and tender, 20 minutes.)
Combine the vinegar, sugar, salt and kombu in a small saucepan. Cook over low heat, stirring constantly, until the sugar and salt dissolve. Remove from the heat and let cool.
Meanwhile, transfer the rested rice to a large wooden salad bowl and delicately spread into an even layer using a silicone spatula. Let the rice rest again for about 15 minutes undisturbed.
Slowly drizzle most of the kombu vinegar all over the rice and gently fold together, without smashing any grains of rice. You can press any large lumps apart and fold the rice back into the vinegar. Taste the rice and, if desired, add more of the kombu vinegar. Continue to press the rice apart until no lumps remain and the grains are uniform and shiny. You can keep the sushi rice covered in a large Dutch oven lined with plastic wrap until ready to use. Makes about 6 cups.