Transfer freshly cooked or warm rice to a large bowl and season with salt. Mix well and cover with a lid to keep warm.

Strain the canned tuna of its liquids using a fine sieve and mix with mayonnaise and sriracha sauce in a small bowl. Set aside.

Spread out a large sheet of plastic wrap on a clean surface. Moisten the rice paddle and divide cooked rice into four equal portions.

Scoop one portion (¾ cup) of rice into the center of the plastic wrap and spread it out evenly into a circle, about ½-inch thick.

Divide the spicy tuna mixture into four equal portions and scoop one portion into the center of the flattened rice.

Lift and twist the corners of the plastic wrap to form a circular rice ball, gently flattening it to about 1.75 inches thick and shape it into a triangle.

Cut a half sheet of nori into vertical strips. Remove the triangle rice ball from the plastic wrap and stick the rough side of the nori strip on the base of the ball. Garnish with black sesame seeds.