Wash and soak the brown rice in water for at least 1+ hour. Cook in rice cooker according to directions (I typically do 4 cups water to 2 cups brown rice). Remove from rice cooker and mix in rice vinegar. Set aside and allow to partially cool.
Meanwhile heat the oil in a pan, and add the shrimp. Cook 1-2 minutes each side, or until browned and cooked through. Remove to a paper-towel lined sheet and allow to cool.
Keeping each ingredient separate, dice the avocado, cucumber, and shrimp and set aside.
In a small bowl, mix together mayonnaise, sriracha, and soy sauce and set aside.
Begin assembling the shrimp stacks. To get perfect circles use a cookie cutter or biscuit cutter and layer ingredients inside. For an easier option, just build a bowl with the ingredients. Layer rice , then chopped shrimp, and top with a mixture of diced cucumber and avocado.
Top with sriracha mayo and sprinkle with sesame seeds and/or scallions as desired.