Heat the oil in a large heavy pot over medium heat.  Add the leeks and cook, stirring occasionally, until they are softened, about 5 minutes.  And the corn kernels, reserved cobs, vegetable stock (try the Roasted Vegetable Broth) and potato.  Season lightly with salt and pepper, increase the heat to high and bring to a boil.  Reduce the heat to low and simmer, covered with lid slightly ajar, until the vegetables are soft, about 35 minutes.

Discard the cobs and let the mixture cool slightly.  In a blender, in batches, purée the soup until totally smooth. (When puréeing, vent the lid or remove the cap and hold a kitchen towel in place over the opening.  Don't fill the blender more than half full.)  Place the soup in a fine mesh strainer (again, in batches) set over a tall bowl or pot, and strain, discarding the solids that remain.  Chill the soup until cold, preferably overnight.  If you think the soup is too thick when you are ready to serve, thin it by adding cold water in ¼-cup intervals.  Stir in lemon juice and season with salt and pepper.  Top each serving with crème fraîche and some of the chopped chives.