Central Bank.  So the EU is a union of states by the treaty of Lisbon. Euro, is a separate treaty (Maastricht) Not every union member is a Euro member.  However, a Euro member has ceded it's minting authority to the ECB. And the ECB simply does what it takes .....an ECB EXIT strategy is not foreseen. That is why the kicking in of Brexit was fortunate for the UK. Coincidentally fortunate.  I would almost be seduced to suppose the EU......