Prepare the barbecue (or heat the oven grill). Blanch the sweetcorn whole in salted boiling water for 1 minute, then place in an ice-bath to stop it cooking any more. Drain and pat dry.
In a small bowl, mix together the butter, shio koji, miso and chilli until smooth and evenly blended. Alternatively, pulse together in a small food processor. Gently melt the butter in a saucepan.
Place the sweetcorn on the barbecue rack, directly over the hot coals, to colour lightly on all sides, then use a pastry brush to liberally coat with the miso koji butter. Continue cooking until the sweetcorn is nicely blistered all over. Give it one final brush of the butter, then serve.