I’ve done that, also a couple chips and a couple mini marshmellows. I’m not a big bittersweet chocolate fan, tho, so in my pre-cholov yisroel days I used to use a hershey’s kiss. I have also done chocolate chips and a glob of peanut butter.
I also use chocolate chips, but I melt them first with some soy milk and honey. It’s richer that way, and also I just don’t like the look of it when it comes out of the oven and you can see the individual chocolate chips in the center. I think it looks nicer when it’s a homogeneous filling.