Is Nibbana a state of mind or an element (dhamma)? I have four parts of this question,


Is Nibbana a state of mind or a dhamma?
If Nibbana is a state of mind, is it merely the uprooting of craving?
If the answer for the second question is "Yes", then why it's said "Nibbana is the object of eight super-mundane consciousness? Or simply, how would a stream winner experience/see Nibbana with the existence of craving?
"One attain Nibbana by uprooting craving" or "One uproots craving by attaining Nibbana" Which one of the aforesaid is correct?
Nibbana is asankhata dhatu (unconditioned element or unconditioned dhamma) therefore it is not a state of mind (MN 115).

Insight wisdom (vipassana) into the three characteristics uproots craving (SN 22.59).  

Nibbana is a sense object (ayatana) of mind consciousness (Ud 8.1) but it is not a type of consciousness. This is why 'mind-objects' are called 'dhamme'; because they also include Nibbana. 

It is because Nibbana is not a state of mind that a stream winner can taste the peace of Nibbana; even though their mind is not completely pure. This can be compared to feeling a cool breeze on a warm sunny day. Because the breeze is one thing &amp; the sun is another thing, both the cool breeze &amp; warm sun can be experienced together. Similarly, the cool peace of Nibbana (experienced from non-attachment) can be experienced by the stream-winner, even though the mind is not completely pure &amp; cooled. 

This said, when a stream-winner is in the stream, their mind is without asava (MN 117). In other words, it is inaccurate to say a stream-winner has craving; as though they always have craving. A stream-winner only has craving when their mindfulness has lapsed. 

In conclusion, the mind attains Nibbana by using Path concentration &amp; insight wisdom to uproot craving. The suttas say the Path (of concentration &amp; insight) is conditioned. Therefore, the mind does not uproot craving by attaining Nibbana. First craving is uprooted using wisdom and what remains leftover is experiencing the peaceful element of Nibbana. SN 22.59 says: 


  Seeing thus [with insight wisdom], the well-instructed disciple of the noble ones grows disenchanted with form, disenchanted with feeling, disenchanted with
  perception, disenchanted with fabrications, disenchanted with
  consciousness. Disenchanted, he becomes dispassionate [uproots craving]. Through
  dispassion, he is fully released [Nibbana]. With full release, there is the
  knowledge, 'Fully released.' He discerns that 'Birth is ended, the
  holy life fulfilled, the task done. There is nothing further for this
  world.'