This is probably a good example of "movie magic," meaning that they used different capes for different scenes, to create exactly the kind of effect they wanted in a given scene. We know in the film that his cape is tailored to be shaped like a bat wing (as seen in the scenes of him gliding through the city). But in this particular scene, they likely used a larger cape so that it would wrap around him to create that "cloaked in darkness" image.
So if you are looking to recreate this particular image, you can still use a cape cut in the shape of a batwing, but just make it about 1 foot longer than your height, and about 6 inches wider. That way it will drape the floor and wrap around you.