Break each of the cobs into 2-3 pieces, put them in a large saucepan and add 1.5 litres (2 3/4 pints) water. Bring to the boil then reduce the heat and simmer, covered, for 1 hour. You will need to skim the foam from the top of the liquid with a large spoon, especially for the first 30 minutes. Strain the stock through a fine-mesh strainer or cheesecloth.

Heat the oil in a clean saucepan and add the ginger, shallots and chilli. Fry gently for 5 minutes, stirring frequently, until the shallots are softened. Add the kaffir lime leaves or lime zest, reserved corn stock (or the equivalent volume of vegetable stock) and coconut milk. Bring up to the boil, then reduce heat immediately and simmer for 5 minutes. Add water, if necessary, to achieve the texture you prefer. Add the reserved kernels and simmer for another 5 minutes.

Ladle the soup into warm bowls. Stack about 6 basil leaves together, roll them into a cigar shape and slice thinly. Repeat with the remaining basil. Scatter the basil strips over the soup. Serve with wedges of lime.