Cook the corn cobs in boiling salted water for approx. 10 mins, drain. Remove the corn kernels from the cobs using a sharp knife.
Heat the oil in a pan. Sweat the onion and garlic for approx. 7 mins. Add the spices and cook for a further 1 min. Pour in the water, bring to the boil, season with salt. Stir in the polenta. Reduce the heat, simmer for approx. 6 mins. over a low heat, stirring to form a thick paste, allow to cool slightly. Mix in the cornflour, butter and egg yolk with the reserved corn kernels, leave to cool.
Using your hands, shape the mixture into approx. 30 nut-sized balls. Empty the breadcrumbs onto a plate. Toss the balls in the breadcrumbs, press down firmly. Fill a pot to 1/3 with oil and heat it to approx. 175°C. Lower the croquettes into the oil in batches using a slotted spoon, fry for approx. 3 mins. all over. Remove, drain on kitchen paper and keep warm.
Mix the sour single cream with all the other ingredients up to and including the salt. Drizzle the sauce over the warm croquettes, garnish with the coriander.