How to Make It
Roast the jalapeño on a skewer directly over a gas flame or in a pan under the broiler, turning, until charred all over. Transfer the jalapeño to a bowl, cover with plastic wrap and let steam for 10 minutes. Peel, core, seed and finely chop the jalapeño. In a small bowl, combine the jalapeño with the vinegar and sugar and stir until the sugar dissolves.
Melt the butter in a large, heavy saucepan. Add the onion and cook over moderately high heat, stirring, until lightly browned, 6 to 7 minutes. Add the corn and garlic and cook, stirring, until the corn is lightly browned, about 5 minutes. Add the stock and simmer until the corn is tender, about 15 minutes. Season with salt and pepper.
Using a slotted spoon, transfer 1 1/2 cups of the corn to a blender, add the crème fraîche and 1/2 cup of the cooking liquid and puree until smooth, about 2 minutes. Return the puree to the soup.
Add the tomatoes, scallion and 1 tablespoon of cilantro to the soup and cook until heated through. Ladle the soup into 4 shallow bowls, garnish with cilantro, the jalapeño and a drizzle of the chile vinegar. Serve hot.