Do any countries have a procedure that allows a constituent part of that country to become independent unilaterally?
In the case of Scotland, the power to hold a referendum was granted by the government in Westminster by the Scottish Independence Referendum Act 2013. This is an exception to what I am looking for in that the path to independence was dependent on the UK parliament as a whole granting independence, and not just the unilateral decision of, say, the Scottish parliament in Holyrood.
In the case of Catalonia, the Spanish government declared the referendum unconstitutional, so clearly the ability to unilaterally declare independence was stymied.
I am also not interested in attempts to declare independence through force, or de-facto independence achieved through wide-spread international acceptance.
Do any countries provide a legal pathway for independence that can be taken by a territory unilaterally, which cannot be halted or vetoed by central government?