There's a northern English blokeness about me that struggles with adding 'daft stuff' to beers.
A sort of reinheitsgebot based on stubborn northern prejudice. ("It's not a piggin' cocktail!"
So I've never added cocoa or cacao nibs or such like to a beer. I've made beers that taste of chocolate with chocolate malt though. I wouldn't want beer to taste of actual chocolate, or of Terry's Chocolate Orange, personally though. You can just buy a chocolate orange for that! (Currently half price, only £1 at Tesco til Nov 27th!). If I was going to try something it would probably be cacao nibs.
As far as the orange goes, I made a stout with a mate, who is also very northern but is into Belgian beer in a big way, earlier this year, that we put bitter orange peel in, but the orange got lost in the roast flavours. So I'm not sure how you would go about getting the quantities right in order to get the right amount of orange flavour in a dark ale. Maybe you make a tea, or vodka tincture, and keep adding it at the bottling stage until you get the right amount of orange flavour?
Not much help, sorry!
Lob a Terry’s in the FV.