Speaking from making fudge, I have gotten the most consistent results using superfine / castor sugar.
- I haven't noticed starch related issues from powdered sugar, but your mileage may vary; the problem I have had with it is due to clumping in spite of the starch (particularly in a frosting recently).
- I have used granulated sugar to make fudge and it is grainy. I melted most of it, but got sick of stirring it as it got crazy thick. I suppose it's possible to get it to the point of a completely dissolved syrup, but that would take longer than it is worth since you are likely going to run the risk of scorching.
Relative to the taste and structure, you won't get anything special from the caster sugar. However, you will get a more predictable product, and less hassle waiting for it to dissolve. You will also avoid texture problems and reduce the risks brought on by extended time in the pot (i.e. too much heat, likelier to accumulate moisture randomly).
I am assuming your couverture was real chocolate, since you haven't said.
While I don't know the effect of alchohol on chocolate, small quantities of water can easily seize chocolate. It becomes a nasty, pasty, stiff mess.
Typical 80 proof vodka would be 40% alcohol by volume, and so approximately 60% water, so your homemade extract would have had significant water in it.
According to Harold McGee as related in The Kitchn, this sounds like what happened:
The process of refining cocoa beans into chocolate gets rid of all the
moisture, and so the final product is actually incredibly dry.
Technically, even melted chocolate can be considered a 'dry'
ingredient despite its liquid state.
For this reason, adding water to melted chocolate has the same effect
as adding water to flour--it turns into a paste. Food science Harold
McGee explains that "the small amount of water acts as a kind of glue,
wetting the many millions of sugar and cocoa particles just enough to
make patches of syrup that stick the particles together..."
You may be able to still use the chocolate for ganache by adding cream, or adding more water so it becomes smooth again, and use it as sauce... but it almost certainly cannot be recovered for covering confections any more.
To prevent this from happening in the future, don't get your couverture wet--from any source.
Note that the flavoring in truffles is normally in the ganache filling, not the couverture; you haven't said you are making truffles, but this gives you a hint:
One way to keep the couverture dry and still have your mint flavor is to add your mint flavoring to the filling.
Another would be to make your homemade mint extract with grain alcohol rather than vodka, so that there is no water in it.
Best Answer
If the texture changed immediately, then that sounds like seizing to me. Chocolate shouldn't seize if you add fat (oil) to it, particularly if you add it after the chocolate is fully melted. Only water will cause that.
Olive oil varies significantly in purity and quality, so cheap olive oil may actually be worse for this application than a purer, cheaper cooking oil (such as peanut or corn oil). There may have been impurities or even trace amounts of water in the bottle. Or it could be that you just tried to melt it too fast, or you accidentally got some water in there from some other place (condensation is a common source).
If you melted it directly in a pan, as in your comment to Kyra, and continued to apply heat after adding the oil, then it's also possible that it didn't really seize at all and that you just tried to melt it too fast.
If it did seize, then there is a cure - perhaps counter-intuitively, adding more oil could help you. Using roughly a tablespoon for each ounce of chocolate will help smooth out any lumps. Add and whisk it very gradually. Of course, this will only work if the oil itself is reasonably pure.
Note: In this case, I would guess that the use of low-quality chocolate isn't your problem if you successfully melted it (albeit to a thick consistency) before adding any oil. Significant impurities in the chocolate itself would lead to seizing during the melting process.