I was trying out the same shot but was unable to achieve such lighting on the subject. Also My background becomes visible If I start playing around with the exposure. i.e. if my exposure is balanced I can see the background, if I under expose my shot the subject becomes dark. What kind of settings are used to get a snap like this ?
It is hard to tell with this small version, but the image might be a composition of two images. If there were several candles lighting the person I would expect multiple shadow lines, eg. from the arm. One could also have a look at the catchlight in the eyes in a high-res version.
To get the background dark enough, in addition to the other suggestions, you need to shut out all other light. You can expose a shot without the candles for the highest exposure to know what your limit is. Then you can add candles to illuminate the subject as much as needed. the background should almost not be affected since the light intensity falls off proportional to 1/r².