@Missingbird Well, depends when, and how, and what. If you add a bunch of cocoa powder, or like hot chocolate mix or something, it could screw up the chemistry, texture and such. Make it dry or hard or heavy. Melted chocolate could do funny things too depending how you add it. Chunks or chocolate chips as is, sure. But it kind of looked like he had something powdered there.
When it comes to baking IMO it's a good idea to at least the first time you make something, just slavishly stick to the damn recipe. If you want to make changes, do it after you know how it comes out. If you want something with chocolate in it, find a recipe for a thing with chocolate in it and stick to that. Baking is different from most cooking that way--If you're gonna fry something up you can do jazz with the ingredients and sauces, just put in whatever seems like it'll be good together, but if you're gonna bake something it'll go weird if you fuck with it.