Some of the better quality cocoa powders won't be as bitter, but they will still be bitter. But since you are putting them on sweet truffles, it balances, but that's just my opinion. I like the contrasting taste, but I try to get the best quality cocoa powder I can.
They actually use the regular 'ol bitter cocoa powder. You don't really get a taste of bitterness biting into the truffle.
There are two main types of cocoa. The first is alkalized or dutch process also called dutch cocoa and the other is natural cocoa. Alkalized cocoa is lighter and not as acid, so it may seem less bitter. Hershey's and most American brands are natural (though I think Hershy's now has one that is half and half) Droste and Rademaker are alkalized.
BTW, some recipes specify which kind of cocoa to use. For example, recipes that use natural cocoa often have some baking soda added to neutralize the acidity of the cocoa. If you use alkalized cocoa in those recipes, the baking soda has nothing with which to react and the recipe doesn't work. Just an interesting bit of trivia.....
Hmmm... I wonder how mixing ps and cocoa would work? It will be lighter but it would add a little sweetness.
I use the Equagold Dutch Cocoa - as seen in this link.
http://www.equagold.co.nz/p_cocoa.php
Absolutely beautiful it is - bitter sweet - so fine it is like talcom powder - and just dissolves on the tongue...as all good cocoas should, without leaving any residue.
To feel it is like touching powdered silk - if there is such a thing 
Best way i can describe it.
Can't breath too heavy around it, other wise it will blow away - 
Bluehue 
This thread is making my mouth water ahhhhg...dangerous thread...
If you want to get away from the bitterness (although I LOVE the taste of natural cocoa powder on truffles) Ghirardelli makes a cocoa and ground chocolate (sweet) mix that might work well.
My sister bought that once by accident when I asked her for cocoa powder, so we used it on our Crown Royal truffles that Christmas. It was wonderful.
There are different types of cocoa depending of origin. Cocoa powder itself is bitter, but the aromatic varieties like the Ecuadorian cocoa have a more floral aroma and nutty taste.
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