Here is an excerpt from a discussion of Denny Conn's Bourbon Vanilla Imperial Porter, one of my all time favorite recipes to brew.
Once fermentation has slowed down, it's time to add your extras (Bourbon, Vanilla, and Oak (if you want)). Denny's original recipe calls for you to split 2 fresh vanilla beans, scrape the insides, chop the pods into quarters, and add all this to the secondary fermenter (or just throw it in the primary if you do not use a secondary.) Then taste this periodically for between 7 and 14 days to get the right level of vanilla flavor you are looking for.
When the vanilla is right and it's bottling/kegging time, it's time to add the Bourbon. Denny's original recipe calls for 10ml. of Bourbon per pint of finished beer. He arrived at this by adding ml. samples to 4 oz. of beer and then scaled up. To some, this may be a bit too much bourbon, so I suggest you do the same calculations and figure out what suits your taste best. You can get more bourbon in, but you can never take it back out
For the sake of math, however, 10 ml. Bourbon/Pint of Beer = 400 ml. Bourbon/5 Gal. of Beer = 1.69 Cups Bourbon/5 Gal. Beer
Although the original recipe does not call for oak, I like it. Restraint, however, needs to be practiced here. Jamil Zainasheff on The Brewing Network recently recommended 1 - 2 oz. of Medium Toast Oak Cubes per 5 gallons of beer. If you are using oak chips, they have more surface area and will require less to get the same effect. Jamil also recommended boiling the chips or cubes in a bit of water for about 10 minutes to sanitize them. Alternately, you could also soak the chips in the bourbon for a bit then pour the whole thing into your secondary fermenter.
