1. 1. Rinse sushi rice under cold water until water runs clear. Cook rice with 2.5 cups water per package directions. 2. While rice cooks, combine rice vinegar, sugar, and salt in a small saucepan. Heat gently until dissolved. 3. Transfer cooked rice to a large shallow bowl. Pour warm vinegar mixture over rice and fold gently with a rice paddle. Cool to room temperature. 4. Flake or finely dice imitation crab. Mix with mayonnaise, sriracha (if using), and soy sauce. 5. Thinly slice avocado and cucumber. Cut nori into small pieces or strips. 6. Lightly oil a ring mold (or round cookie cutter). Place on serving plate. 7. Layer in mold: first a 1/2-3/4 cup seasoned rice (press firmly), then cucumber slices, then crab salad (press gently), then avocado slices, then a thinner top layer of rice (press again). 8. Carefully lift mold straight up. Repeat for additional stacks. 9. Garnish each stack with sesame seeds and nori pieces. Serve immediately with extra soy sauce, pickled ginger, and wasabi on the side.