Pour chicken stock into a soup pot and bring it to a boil. Add the potatoes and cook them, covered, over low to medium heat for 30 minutes. Blend the soup in a blender and then pour it back into the soup pot. Add the milk, butter, cheese, salt and pepper to taste. Mix well and continue cooking until all the cheese is melted. Place a slice of toast at the center of each soup plate. Pour the hot soup on top of the toast and serve immediately. ----