Grease a 20cm square cake pan. Line base and sides with baking paper, allowing a 3cm overhang on all sides. Rinse and drain sushi rice 3 times or until water runs clear. Place rice in a sieve over a bowl. Set aside for 10 minutes to drain.
Place rice and 1 ½ cups of cold water in a saucepan over medium heat. Cover. Bring to the boil. Reduce heat to low. Simmer, covered, for 12 minutes or until water has been absorbed. Stir in mirin. Spread rice over a large baking tray, cool for 30 minutes.
Meanwhile, combine mayonnaise and Fountain Sweet Chilli sauce in a small bowl.
Trim nori sheets to fit prepared pan. Place 1 sheet shiny side down, in base of pan. Spread mayonnaise mixture across centre of rice. Place chicken, lettuce, avocado and cucumber in single layers over rice. Gently spoon remaining rice over, using the back of a wet spoon to spread evenly. Place remaining sheet, shiny side up, over rice. Press firmly to secure. Cover with cling wrap and refrigerate for 2 hours or until set.
Carefully remove slice from pan. Cut into 16 squares. Serve with Fountain Soy Sauce.