Cut each nori sheet into 4 squares, for a total of 24.
Stir rice vinegar into the cooked rice.
In a medium bowl, whisk the kewpie mayo, sriracha, soy sauce and sesame oil until smooth.
Cut the imitation crab meat into smaller pieces. Peel, seed and dice the cucumber. Peel, pit and dice the avocado.
Fold the imitation crab, diced cucumbers and diced avocados into the mayo mixture.
Place 1 Tbsp. of sushi rice into the center of each nori square. Press the rice-topped nori into the muffin cup. Using wet fingers, gently press the rice down into the muffin cup and up the sides of the nori square.
Place a spoonful of the crab mixture into each sushi cup. Top with chopped scallions and sesame seeds, if desired. Remove sushi cups from muffin pan when ready to serve.