1. Fold a kitchen towel and place it inside a wide metal bowl. Use one hand to place an ear of corn upright atop the towel, and use a sharp knife to slice off two to three rows of corn kernels at a time by sliding the knife down. The closer you can get to the cob while slicing, the better. Repeat with all the corn. Reserve the cobs.
2. Place the cobs in a large stockpot and cover with water and bring to a boil. Reduce the heat and simmer for 10 minutes. Then take off the heat and keep covered for 10 minutes. Discard the cobs and reserve the corn stock.
3. Meanwhile, heat another pot over medium heat and add the butter. Once the butter is melted, add the sliced onions and sauté until completely soft and translucent.
4. Add the corn and the chilli and turn up the heat, sautéing just until the corn turns a more vivid shade of yellow. Add just enough corn stock to cover the corn, turn the heat to high and bring to a boil. Reduce the heat and simmer for 10-15 minutes with a lid on until the corn is tender. Strain out the chilli at this point if you prefer it less spicy.
5. Using an immersion blender, carefully blend the soup till it is completely smooth. Strain through a fine strainer.
6. Adjust the consistency using the remaining corn stock and stir in the lime zest.
7. Season to taste with the salt and sugar.
8. To serve, place half a teaspoon of the lime juice in a bowl and ladle the corn soup over.

Garnish with the chopped cilantro and serve hot. Alternatively, this soup can also be served chilled.