1. In a heavy 3- to 4-quart casserole, melt 4 tablespoons of butter over moderate heat. When the foam begins to subside, add the endives and leeks, and stir until they are evenly coated with the butter.
2. Reduce the heat to low, cover tightly, and cook for about 10 minutes, or until the vegetables are soft and translucent but not brown.
2. Stir in the potato, salt and pepper, pour in the milk, and bring to a simmer over moderate heat, stirring from time to time.
2. Reduce the heat to low again and simmer uncovered for about 45 minutes. When the soup is finished the potatoes will have dissolved almost completely to create a light puree.
3. Taste for seasoning, swirl the softened butter into the soup, and serve at once from a heated tureen or in individual soup plates.
