I have brewed two beers so far that I have used cocoa powder; however, I have only drank one of them so far. The one that I tried was my Strawberry and Chocolate Wheat. I did a split batch, and did compared the Strawberry Wheat to it just to see how it turned out. I honestly did not get my anticipated chocolate flavor either from the 100% Unsweetened Cocoa Powder even though I added it into the secondary along with the strawberries. I only got a tint of brown in color, and a hint of low residual sweetness from the Chocolate. Although I have never tried your method of adding chocolate to a stout, I would think that you probably pulled the same thing out of yours too. Next time I suggest just adding chocolate malt to the grain bill, and adding the cocoa nibs if you would like.
What were your flavors that you got from the beer, and also what were the hops that you used? Did the hop flavors come across like you were wanting?