Heat the barbecue or a grill to medium-high. Mix the sake, mirin, miso and sugar in a bowl. Halve the aubergines lengthways and score the flesh on the diagonal, at about 2cm intervals. Brush the aubergines with sesame oil and season with a little salt. Grill for 2-4 minutes, until starting to turn golden and soften. Brush all over with the miso paste. Grill again for another 15 minutes. Keep turning the aubergines every few minutes to stop the sugar from catching, brushing with more of the miso paste when you turn them. The aubergine should be very soft all the way through and coated with the sticky glaze, which is just starting to catch at the edges and turn golden brown.