Zucchini Basil Soup with Crème Fraîche and Pickled Corn

medium corn cobs

small shallot, peeled and finely diced

fine sea salt

champagne vinegar

medium leeks (500 g)

butter

zucchini (about 6 small / 570 g), trimmed and chopped into roughly ½″ pieces

medium yukon gold potato (200 g), peeled and chopped

salt, more as needed

Freshly ground black pepper

water

juice of ½ a lemon

leaves from 1 large or 2 small bunches basil (2-3 cups lightly packed), plus a few pretty sprigs for garnish

crème fraiche, for serving