Rinse the sushi rice several times in cold water until the water runs clear. Drain the rice.
Add the rice to the rice cooker with 2 1/2 cups of water and a pinch of salt. Cook according to the rice cooker's instructions.
While the rice is cooking, prepare the seasoning by mixing together the rice vinegar and sugar in a small bowl.
When the rice is done cooking, transfer it to a medium-sized pot and add the seasoning. Mix gently to combine.
Allow the rice to cool slightly until it's cool enough to handle.
Wet your hands with water and sprinkle a little salt onto them. Take a small handful of rice and form it into a triangle shape.
Make a small indentation in the center of the triangle and add your desired filling.
Add more rice on top of the filling, and use your hands to shape the onigiri into a triangular shape.
Repeat with the remaining rice and filling.
Roll each onigiri in roasted sesame seeds and wrap a small piece of nori around the base of the triangle.
Wrap each onigiri tightly in plastic wrap to keep its shape and store in the fridge until ready to eat.