- Rinse the sushi rice under cold water until the water runs clear. 
- In a medium saucepan, combine the rinsed rice and water. Bring to a boil, then reduce heat to low, cover, and simmer for 18-20 minutes until the rice is tender. 
- Remove from heat and let it sit, covered, for 10 minutes. 
- In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Gently fold this mixture into the cooked rice. Allow the rice to cool to room temperature. 
- In another bowl, combine the sriracha sauce and mayonnaise to create a spicy sauce. Add the cooked shrimp and toss until well coated. 
- To assemble the sushi stacks, use a round mold or a small cup. Start by layering a spoonful of sushi rice at the bottom, followed by a layer of spicy shrimp, then a layer of avocado and cucumber. Repeat the layers until the mold is full, finishing with a layer of sushi rice on top. 
- Carefully remove the mold and sprinkle sesame seeds and chopped green onions on top. 
- Serve immediately with nori squares on the side for wrapping.