Actually, baking powder doesn't create softer cookies, but makes them a little puffier, so, they may
not bake as quickly as flatter cookies, ie, keeping them a little softer/less well done. But, I'm not sure if this is 100% accurate - I'm no expert by any means - will have to do more research.
The soft texture in cookies actually has more to do with what kind of sugars and fats are being used. For example, brown sugar will yield a softer, more moist cookie than white sugar, etc. Also, I believe melted butter yields a chewier cookie than just softened butter. Did you use melted butter? Also, many biscotti recipes don't use any fat at all, so I wonder if they're harder than the butter ones?
Edited to add:
Here's some info I just found from Alton Brown regarding chocolate chip cookies:
For soft cookies:"If your tastes run to the soft and cakey, use cake flour, ***baking powder*** rather than soda, and shortening instead of butter. Chilling the batter and scooping on the small side will add to the puff factor, too."
So this kind of reiterates what I suspected about using baking powder.
For Chewy Cookies:"...chewiness calls for melting the butter, holding back on the egg whites and using more, if not all, brown sugar."
I've added a link on the info from Alton Brown.
Valrhona is a Dutch Processed cocoa, as are most, if not all European cocoas, so the baking powder made no difference as far as reacting with the cocoa is concerned.
I'm glad they turned out great! Now you've gotten me in the mood to make some! Which recipe did you use?
http://www.americastestkitchen.com/ibb/posts.aspx?postID=227815