What should I use for bittersweet chocolate?

Cook's Illustrated likes Ghirardelli 60%; I personally find it has a bit of a sour/bitter undertone, and I prefer Hershey's Special Dark (CI's second choice). Note that chips have added stabilizers to help them keep their shape under heat (so they don't ooze when making cookies); bar chocolate is always preferable for applications that don't need chips.

Additional chocolate recommendation: Affordable and easy to melt Ghirardelli 60% chips. Unlike the Nestle/Hershey's, these chips are smooth and melty and have not been tempered to death and adulterated with other substances (a common practice for cheap candy to increase its heat tolerance). Happy baking!

I think any good bittersweet chocolate (~50-65% cacao) will work for both the cake and the icing, regardless of form (chips vs. bars). You're melting the chocolate, so chips or bars is not that important (though chips are just easier to melt, no chopping). I wouldn't go crazy spending $$$ until I've done a test run with a less expensive chocolate, though I would keep the % cacao at around 50% or higher. Sometimes, it tastes just as good with less pricey chocolate. And Trader Joe's Pound Plus dark chocolate is ~55% cacao and it's ~$5.