Cook the corn cobs in boiling salted water for 5 minutes, then drain and dry thoroughly with kitchen paper.
Toast the corn over a hot barbecue or griddle pan for 8 minutes, turning regularly.
Brush the corn with melted butter and scatter with coriander.
Crumble over the queso fresco and sprinkle with cayenne.
Serve with sour cream for dipping and lime wedges for squeezing over at the table.