Rinse the sushi rice under cold water until the water runs clear. Cook in a rice cooker or a pot according to package instructions. Once cooked, transfer the rice to a large bowl, season with salt, mix well, and let cool to room temperature.
In a small bowl, mix the drained tuna with Japanese mayo and sriracha sauce. Set aside.
Prepare a 15 x 15 inch piece of cling film on a clean surface. Moisten a rice paddle with water.
Divide the rice into four equal portions. Place one portion on the cling film and flatten into a ½ to ¾ inch thick circle.
Place a portion of the spicy tuna mixture in the center of the rice.
Lift the corners of the cling film and twist to form a ball, ensuring the tuna filling is enclosed in the center.
Gently press down to flatten the ball into a 2-inch thick disc, then shape into a triangle using the L-shape of your thumb and index finger.
Cut the nori sheets as desired and wrap a strip around the base of each rice ball. Garnish with black sesame seeds if using.