I am skeptical of adding chocolate anything to a brew in the boil or at any time during primary fermentation. The first time I attempted to add around a pound of cocoa powder to a 23 L batch of chocolate orange stout, I had zero fermentation activity and the batch ended up being dumped. Came to read some indications online that cocoa contains some anti-fungal agents that can inhibit the growth of yeast. Go figure.
My second go-around, I added the cocoa at the start of secondary fermentation, by making a hot slurry as you describe and pouring it into the carboy. Turned out fantastic. I'm of the opinion that this is the way to go. The yeast is firmly established at this point, so no worries about fermentation problems. I am also told that cold extraction in the presence of alcohol helps to draw out the flavours better, whereas flavour might otherwise be boiled and/or bubbled away if added during boil or primary fermentation