A home brew recipe question?

Thinking about making an ale with Hazelnut extract & cocoa. has any one tried this combo? Whats the best time to add them? Boil, secondary, or at bottle time? I brew a lot of ale, but mostly just malt,grains,hops & I've never tried adding other things like the above so not sure? Remember having a brew a few years back that had Hazelnut & Vanilla & it was great! Thinking pure cocoa would be the even better! Any thoughts?