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
I had to do some reading on this one. There are a lot of answers.
1- You should find a better recipe. Chocolate spread recipes I have used come in one of two styles.
The water in your recipe seems to be a problem. The nutella style that uses fat, sugar, and chocolate. These will not really dry out. Sometimes the oil will separate a little and will need to be mixed back in. I would group frostings in this group although, of course, they contain far more sugar.
See this recipe:
http://www.foodnetwork.com/recipes/gale-gand/chocolate-hazelnut-spread-with-crepes-recipe/index.html
Thick chocolate candy. AKA fudge. These contain water but the sugar and water are cooked together into a syrup so the water is not able to immediately dry out. They will crystallize if they are badly handled- many recipes contain a little corn syrup to prevent this. This does not seem to be what you are trying to make.
2- Store your concoction in a lidded container that isn't too much larger than your sauce. I use mason jars to store mine. Alternatively you can push plastic wrap down onto the surface of the spread.
3- Refrigeration. Recipes that don't contain dairy don't need to be refrigerated and in fact shouldn't be because it causes them to harden.
4- Adding water to melted chocolate is only a problem when dealing with chocolate that is dry and meant to solidify such as a molded chocolate. In such a case even a drop of water will cause seizing. In this case where there is already quite a bit of water in the recipe seizing is not a risk and you shouldn't have any problem mixing a tiny bit back in. Do not add water to the nutella-like spread recipes. They do not contain water and it will not mix in.
I can't tell from your description what is causing your sauce to harden- it could be drying out, separating, or crystallizing. Any of these will be solved by reheating and thoroughly stirring. Although water could be added to restore dried syrup I think you will get better results by switching to a recipe that uses fat in place of the water.