If one buys the oil expeller for making cocoa butter, which bean type would you recommend that might counteract to a small degree the astringency/metallic taste that I am getting with my chocolate.  I realize there are other factors that are contributing to this taste, most probably, but I thought it might be a place to start.  I was not thinking of making single origin chocolate, just a less sharp tasting chocolate and thought the cocoa butter might be one thing to address. 

This is not the direction I would take for addressing that flavor you are getting.   Especially since it is only a small degree of off flavor.   To my knowledge, there isn't a lot of chemistry that goes on with cocoa butter. Or at least once it is formed.

Generally speaking, there are three broad ways to affect the flavor of chocolate.  Not a lot different from maths.  You can add things.  You can subtract things. And this is where it kind of isn't a perfect analogy; you can alter them.  This third way is what you are asking about and is also what I was referring to as chemistry that goes on.

You know about adding flavor.  You put in some vanilla and you taste vanilla.  It does not change any of the other flavors.  Sometimes with enough it masks other flavors.  But it does not remove them or counteract them.

You know about subtracting flavor.  During the first 12 hours in a melanger lots of acids leave a chocolate.  You can smell them.  They are leaving.

Then there is the chemistry.  When you roast you actually make chemicals that we recognize as smelling and taste of chocolate.  We didn't add the chocolate flavor no did the removal of other flavors uncover the chocolate flavor.  It was created.

But of course taste isn't that simple.  And really there is a forth way to alter the flavor, and probably closer to what is going on.  And one of the reasons I associate chocolate with alchemy.  It's mysterious and magical...and frustrating at time in the way it can't quite be predicted.

The alchemy is our perceptions.  There is no one chemical that smells or tastes like chocolate.  Really.  What we think of as the chocolate smell isn't one chemical.  What you smelling is a unique combination of sweat, cabbage and beef.  When they are combined we no longer notice or can even recognize those individual aromas.

If you are having trouble wrapping your head around that, look at this picture.

yellow pod

What is the color of that cocoa pod?  If you say yellow, you are both right but also wrong.   You computer screen does not display yellow.  It can only display unique combinations of Red, Blue and Green.  RGB. Sound familiar? I'll grant what you are seeing looks like yellow.  But in reality it is a combination of red and green.  Cabbage and beef as it were.  But you see yellow.  You smell chocolate.

The whole point of this is to say taste is very complicated.  And also not absolute.  What you taste can be very different from what I taste.  I personally don't have a tendency to taste that metallic taste you mention.  Or at least, it does not taste of metal to me.  It tastes more of tangy acidity.  It is down to this complex dance of our receptors and how our brains put it together.

The best analogy is being color blind for certain colors.  If you show the above image to two people who are red and green color blind respectively, the red color blind person will see a green pod and the green color blind person will see a red pod.

green red pod

I am going bring this back around.  And please know I am making this next part up as an example only.  It most likely is totally not accurate, but it gets the point across.  I propose that what I taste as tangy acidity is the yellow pod.  Not a stand alone chemical, but a combination of two.  Red and green.  You on the other hand can only taste one of the two chemicals and the result is the red pod for you - metallic astringency.

With that in mind I think you can see there isn't anything concrete I can tell you to do with 100% accuracy that will change how you perceive that flavor.  Maybe adding a certain cocoa butter will combine with it to make it seem to go away, but most likely I doubt that will be the case.  And in any case I'd have to be psychic to hazard a guess.  The combinations are just to immense.

And the photos point out another issue.  None of these tastes exist by themselves.   Let's pretend the purple in the right photograph is a particular flavor you love in your chocolate.  But you hate the red.  So we do figure out a way to add green to it so that your pod is yellow.  Success!  Right?  Sure, we fixed the red pod and now it is yellow...but that wonderful purple you so adore is not green.  And maybe the green is ok, it isn't purple.

This is pretty close to what happens when you alkalize your chocolate.  It massively reduces bitterness and astringency, but it also lays waste to all the other subtle flavors in your chocolate leaving you with a pretty bland, one dimensional chocolate.

There is a ton more I could say about taste, taste perception and the like but I fear the dreaded information overload and deer in the headlights syndrome. So let's circle back to things you can try.

I can offer a few things.  And unfortunately they come with the disclaimer that I don't know enough about your chocolate or how you roasted it.

Experience tells me metallic astringency can come from over heating your beans.  Scorching them.

But it can go the other direction and come from under roasted beans.

And without knowing how you roasted I can't tell which if either it is.

Finally, it can just be the bean and no treatment will change it.  And this is what I find the majority of the time.  That no matter what, you are going to taste metal when I taste tang.  In that case, your only option is to pick another bean.

As for adding your own pressed butter.  Sure, give it a shot, but at only 5-10% of the recipe, it is the last thing I would try, not the first.  It could well add a flavor to your chocolate but my gut feeling is that it isn't going to miraculous alter that one flavor you don't like.