Yuzu kosho tamari
Rinse the sushi rice under cold water until the water runs clear. Combine the rice and water in a rice cooker and cook according to the manufacturer's instructions. Once cooked, transfer the rice to a bowl.
In a small saucepan, heat the rice vinegar, sugar, and salt over low heat until the sugar dissolves. Pour this mixture over the warm rice and gently fold to combine. Allow the rice to cool to room temperature.
Cut the ripe avocados in half, remove the pit, and scoop the flesh out. Slice the avocado into thin pieces.
Wet your hands to prevent sticking and take a small amount of sushi rice (about 2 tablespoons). Shape it into an oval mound. Place a slice of avocado on top of the rice mound.
Drizzle the yuzu kosho and tamari over the avocado nigiri. Sprinkle sesame seeds on top and serve with nori sheets on the side if desired.