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 and water is absorbed.
Remove from heat and let it sit, covered, for 10 minutes.
In a small bowl, mix the rice vinegar, sugar, and salt until dissolved.
Once the rice has rested, fluff it with a fork and gently fold in the vinegar mixture. Allow the rice to cool to room temperature.
In another bowl, combine the mayonnaise and Sriracha sauce to create a spicy mayo. Adjust the amount of Sriracha to your preferred spice level.
To assemble the 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 mayo, a few shrimp, avocado slices, and cucumber.
Repeat the layers until you reach the top of the mold. Gently press down to compact the layers.
Carefully remove the mold and sprinkle the top with sesame seeds and chopped green onions.
If desired, serve with nori strips on the side for added crunch.