Mash together the butter, honey, ketchup, garlic and some seasoning, beating until quite smooth. At this point, if you are also cooking for children, split the mix into two serving bowls, before stirring the harissa into just one bowl. Add a pastry brush to each butter mix.
Wrap each corn cob in a double layer of foil. Barbecue, turning, for 30-35 mins until tender and a little charred. If you want them more blackened, simply unwrap and throw directly on the barbecue for a few more mins.
Pile onto plates and let everyone butter their own.