Prepare all ingredients and have a bowl of water with a little added vinegar ready to dip your fingertips into. Cover seaweed squares with a damp cloth or cling film to prevent wrinkling. Place about 1 ½ T rice into the middle of each nori sheet. Wet your fingertips and push rice up to the top corner, then make a shallow indentation down the centre. Smear a small amount of wasabi into this hollow. Cover with a prawn, a strip of cucumber and a strip of avocado. Sprinkle with sesame seeds. Fold over one corner of the seaweed. Wet the opposite side then fold and stick to make a cone. Lastly, fold in the end, if you like, to prevent everything from falling out. Serve with soy sauce and pickled ginger.