To be honest? Personal experience chocolate doesn’t bother chickens. When I was a kid, there was an Ancona roo named Flopcomb my grandfather had. I would “fight” him, grab him up, then carry him around for hours and feed him all kinds of things, including copious amounts of Hershey kisses and those small bite size Reese cups. My grandparents always had a dish out on the dining room table. I know those aren’t dark chocolate or anything, but it never bothered him at all. Of course to be safe don’t deliberately feed it to your chickens like I did back then, but I bet your hen will be just fine.