I am going to recreate the Chocolate Cherry Stout I brewed a few months ago. However, this time I plan to add cocoa powder to the boil rather than cocoa nibs in the secondary. How much cocoa powder would I use for a three gallon batch?
I just cracked the first bottle of my bastardized schwarzbier (used an ale yeast instead of a lager, o.g. 1.052), which had two additions of Cocoa and Cinnamon added to the secondary (I boiled the mixture to kill any nasties, of course.. and also added 1 TBSP of sugar to create co2 to re-fill the headspace in my secondary).
Anyhoo, it was a 3gallon batch and I added a total of 4oz dutch-processed cocoa (from Penzey's, very fresh and chocolatey), and there's a strong cocoa aroma and flavor.
But since you're brewing a cherry stout, there may be some adjustments necessary:
-Chocolate has to compete with the roastiness of a stout.. maybe add 1oz
-Chocolate has to balance out with the cherry flavor (add or remove an oz depending on your preference)
-My OG was a mid-range 1.052, if yours is higher/lower you may want to compensate as well
-I added it to the secondary to avoid the primary fermentation driving off flavors/aromas.. so an extra oz might be worthwhile
All that being said, I'd recommend being slightly conservative in the boil, and then taking a sample at the end of fermentation and assessing how much cocoa goodness you'd like, and adjust as appropriate using the boiled slurry/sugar method. It may add an extra week as you'll want the beer to sit on the cocoa for a little while, but your patience will be rewarded.
Good luck!