I used Ghirardelli 60% cacao bittersweet baking chips in my (Dark) Chocolate Orange in the Coconut cookies.
I thought about using a chopped-up bar of dark chocolate, but I questioned whether the chocolate would melt too much and "bleed out" of the cookies as they baked in the oven. These chips were the perfect fix because they brought the dark chocolate flavor and the same meltability as regular semisweet chips.
So there you have it. Problem solved, question answered, and--most important--cookies baked.