Put the rice in a small pan with 325ml (11fl oz) water, bring to the boil and simmer, covered, for 10 minutes. Remove from the heat, leave the lid on and allow to sit for 10 to 15 minutes.
Transfer to a large bowl. Add the vinegar, mirin and sugar, mix again and continue mixing until cool.
Lay the nori sheets on a dry surface and divide the rice between them, spreading it out evenly with your fingers and patting it flat. Dip fingers in a bowl of cold water to help stop the rice sticking to them, and wipe on a clean, damp cloth.
Neaten the edges with a knife, then place the salmon evenly over two of the rice-topped nori sheets, arrange an even layer of chives on top and then sandwich with the other half of prepared nori.
Pat the sandwich together and push the edges straight with the flat of a knife. Cut in half and then cut each half into three fingers. Serve topped with chopped chives and soy for dipping.