Dry-fry the sesame seeds in a small frying pan until golden. Leave to cool slightly, then place in a food processor or blender with the butter, chilli, lime rind and juice and salt and pepper to taste and blend until combined. Transfer the butter to a piece of foil and roll into a sausage shape. Chill for 30 minutes, then remove from the foil and cut into thin slices.
Parboil the sweetcorn cobs for 5 minutes, then cut into 1 cm (½ inch) slices. Place on double-thickness pieces of foil, top with the butter slices and wrap up the foil parcels tightly. Cook on a hot barbecue for 20—25 minutes until the corn is tender. Garnish with lime rind and some watercress and serve immediately with more butter slices.