As the King has mentioned, the best way to add chocolate flavor and taste to a brew is to add chocolate extract (alcohol based) or chocolate liquor right at racking, the same time you'd ad priming sugar. When chocolate and/or cocoa is added during the boil, most of it sinks away during the ferment...unless you're adding literal pounds of it, no taste will come through.
For example, make a cup of hot cocoa with water, and let it sit for a week or two. Most of the cocoa has sunk. This is because cocoa and similar mixtures are suspensions when added to water (or mostly water). That is, most of it doesn't dissolve into water, it's suspended until gravity brings it back down.
Cocoa essence and cocoa liquor, by contrast, dissolves cocoa, vanilla, etc into alcohol, where it remains and doesn't separate out. When you add cocoa to the boil, you need to add a ton of it and hope enough remains in suspension for a portion of it to be dissolved into the alcohol once fermentation is complete.
I wrote back and fourth to Young's a bit last week regarding their famous Double Chocolate Stout. They said, in a nutshell, that they add the tiniest amount of chocolate to the boil, just so they can say they did. All the flavor and aroma comes from adding chocolate essence prior to bottling/kegging.
