There was a thread about summoning a stamp for the Darkling City, and another thread about summoning stamps for Burning Land tiles.

Maybe you can create a stamp which already contains your unique resource and summon it? To have it only done once, you could set cooldown to something like 800. (or whatever max game turns is these days).

As a wild guess, I would make a 2-stage improvement that gives the bonus on the "second" stage, the upgrade. Most people won't raze a second-level city.

I have some buildings on the Nexus which give bonus resources you can look at and use as a design template if you want.

You can set <RequiresCityImprovement>1</RequiresCityImprovement> and make a second building which either upgrades to, or depends on the one you want to give the ability to. You can try giving Prereq of a unique improvement.

My final option is to find the spot where the Capital City is hardcoded, and use that to make it Capital Only.

Maybe make a fake resource that the player cannot see and give each player a certain amount of this resource that is required to build the building in question. If there is no way of generating this resource (for example give it to the player when he builds his first city), then he can only build your building exactly once. Unless he removes his entire empire (or knows of the exploit to get a lot of the I can build it resource).

Or think of another way of giving someone this resource that they can exploit, I'm sure there are other ways about it.

Giving a building a unique resource cost shouldn't be a problem, I've done that before.

Nevermind, I found one (magma forge).

And, this works a treat!  I was using the wrong type of tag... I had been using a <ProductionRequirement> as used by UnitTypes (which doesn't work properly).  To get it working you can instead use this on an ImprovementType:

<GameModifier>
   <ModType>ConstructionResourceCost</ModType>
   <Attribute>MySpecialResource</Attribute>
   <Value>-1</Value>
</GameModifier>

For my purpose I then just have a faction trait that grants +1 of MySpecialResource.  You can then construct the unique building.  If you raze it you can't build another one.  Huzzah!

Thanks (again) HF.    

Thanks to everyone else for the quick responses too.  I also tried using a Spell (as suggested by thadianaphena) to create the bonus resource and also got that to work  

So now i've actually got a couple of options at least, yay.

I think the key is to remove your unique resource as a building effect, not as a CostResourceEffect, because when you demolish a building you get back the resources marked as construction costs.

The CoreImprovements.xml has the GuildLendingHouse improvement, that gives 2000 gold just once. Perhaps looking at it could help you. Use that code and replace Gold by your resource and 2000 by -1

Yes, but the important thing about the lending house in this case is that it provides a one time lasting effect. I was just using it as an example of this behaviour. With that code I believe that the unique building will substract one unit of the unique resorce, and this substraction won't be "fixed" after demolishing the building / losing the city. I have not tested it because I am not at home but I am fully convinced it works.

Great! Now I am curious to learn about those faction buildings