Brush the corn cobs with the oil, then season them. Heat a ribbed grill pan or frying pan over high heat. Sear the corn and the whole chillies, turning them often using tongs, so they are cooked all the way ‘round and charred in places. Remove from the heat and set aside to cool.
To make the dressing, whisk the mayo, vinegar, garlic, onion powder and chives together in a bowl.