Put the ancho chilli into a small bowl and pour over enough boiling water to cover. Leave for 10 minutes, drain, then discard the seeds and stem.
Heat the oil in a pan and cook the onion and celery with a pinch of salt for 10 minutes until soft. Add the garlic and cook for a minute, then add the spices and cook for another minute. Add the ancho chilli flesh, peppers and vegetable stock, and simmer for 15 minutes. Season, then use a stick blender to whizz until smooth. Keep warm.
Meanwhile, heat a griddle pan over a high heat, lightly brush the corn cobs with oil and season. Cook the cobs for 8-10 minutes, using tongs to turn them until really charred. Put onto a chopping board and allow to cool slightly. Turn a cob up on its end, then use a large sharp knife to carefully remove the kernels in a downward movement. Turn and repeat until all the kernels have been removed, then repeat with the other cob.
Stir the lime juice into the soup and add the sweetcorn. Divide between 4 bowls, scatter over the coriander and serve with lime wedges.
Find dried ancho chillies in larger supermarket speciality ranges or buy online.