I have a black stout fermenting right now. I added about 8oz of cacao powder in the boil and I'd like to add some vanilla extract, to taste. Can this be done before bottling? Should this have been added in the boil (for future reference)?
Some 'adjuncts' (vanilla, coffee, juniper, other alcohols) can be added to the end of the boil but I would say most adjuncts should be separately sanitized, pastuerized, or boiled and then added to secondary after transfer from primary rather than added to the end of the boil.
Vanilla extract in particular would experience significant boil-off if added to the boil. It is ~35% alcohol. If you are adding vanilla beans, that's a different story. You need to find another way to sanitize vanilla beans (I'd suggest crush them and mix with vodka).
If you are adding something that is ~30%+ in alcohol content, you probably don't need to sanitize or boil it. That percentage of alcohol should stay bacteria free on its own. That's why that bottle of rum that you opened in 2005 hasn't molded or gone bad.
If you are adding an adjunct that is not alcoholic, you should consider using a method to sanitize it. Your choices, from most conservative (safe) to least conservative: boil for 5 or 10 minutes; pasteurize (place in heated water @ ~160F) for 5 to 10 minutes; soak in vodka prior to addition (overnight is my rule of thumb); spray with sanitizer (I don't like this option). You kind of have to decide how 'dirty' the object is, whether it is going to be crushed up before going into the boil, etc. to decide what is right.
I recently added 3T of pure vanilla to a stout. It isn't ready so I can't tell you how that amount worked out in the final product. However, we recently sampled the secondary and found the vanilla pleasant and not overpowering. I found ~2.5T (tablespoons not teaspoons) as a suggested amount from several websites. YMMV.
Here are some of my rules of thumb: Alcohols: vanilla, grand mariner, rum, bourbon. Don't boil them or pasteurize them! Add them as is, preferably from sanitary containers. If you are paranoid, you can sanitize the lip of the bottle.
Oak chips: soak overnight in vodka.
Honey: pasteurize @ 160F
Fruit is whole 'nother question...
Hope that helps.
I have added flavoring extracts to several beers. I always do it at bottling/kegging time. My advice with extracts is to err to the side of using less. One beer in particular turned out to be almost undrinkable with an overwhelming extract flavor. Do you have a pipette or other accurate method of measurement? Make sure to use it, and write down how much extract you used for future reference.
I made a vanilla stout and I found that 2T of this vanilla extract was too little for a 5G batch. Next time I make it I'll double that amount. The beer was excellent after two weeks of bottle conditioning, but by four weeks the vanilla was almost imperceptable.
I'm in the process of making my first beer that contains vanilla. I bought a pack of 8 beans for $4. I cut two of them down the middle and placed them in the boil for the last 5 minutes. I cut the other six up and partially smashed them. They are soaking in 4.5oz of vodka and I will be filtering them and adding the liquid when I place the batch in secondary. I figured they would fare better than using extract during the boil, so that's why I chose them over using the extract. I'm pretty excited for this batch!