Rinse the sushi rice under cold water until the water runs clear. Combine the rice and water in a medium saucepan, bring to a boil, then reduce to low heat, cover, and simmer for 15 minutes. Remove from heat and let it sit, covered, for an additional 10 minutes.

In a mixing bowl, combine the rice vinegar, sugar, and salt. Microwave this mixture for about 20 seconds until warmed slightly, stirring to dissolve the sugar and salt.

Fluff the cooked rice with a fork and stir in the vinegar mixture. Let the rice cool while you prepare the shrimp.

In a pot of boiling water, cook the shrimp for 2-3 minutes until they turn pink and opaque. Drain and let them cool. Chop the shrimp into bite-sized pieces.

In a small bowl, mix the chopped shrimp with the sriracha and mayonnaise until well combined.

To assemble the sushi stacks, place a sheet of plastic wrap on a flat surface. Layer with a sheet of nori, followed by a ½ cup of sushi rice, spreading evenly. Add a layer of spicy shrimp mix and top with slices of avocado.

Carefully lift the edges of the plastic wrap to form a stack. Compress gently to hold everything together. Remove the plastic wrap, then repeat with the remaining ingredients to create three more stacks.

Slice each stack into quarters or halves with a sharp knife. Arrange on a serving platter and drizzle with extra sriracha and sprinkle with sesame seeds.

Serve with soy sauce on the side for dipping.