Why chocolate chips hold their shape and a chopped bar doesn't (quite)
Chocolate chips are formulated with slightly less cocoa butter and added stabilizers specifically so they keep their shape in the oven instead of melting completely into the batter — that's why chocolate chip cookies have distinct pockets of chocolate rather than chocolate swirled throughout. A chopped chocolate bar doesn't have those stabilizers, so it melts a bit more freely, giving slightly more spread and gooier pockets rather than hold-their-shape chunks. It's a very close substitute in flavor and still gives great results — just a marginally different texture.
When to reach for cocoa powder instead
Cocoa powder isn't a texture match for chips at all — there's no chunk or chip to bite into, since it's whisked directly into the batter with a little sugar and melted butter to restore the fat and sweetness a real chocolate would have brought. This works well when you want an overall richer, more chocolatey batter (think a brownie-style cookie) rather than pockets of melted chocolate throughout. It's the better move when you're completely out of any bar or chip chocolate, not a preferred choice when you do have one on hand.
FAQ
Can I use chocolate chunks or chocolate bark instead of chips? Yes, as an easy 1:1 swap by cup — both behave similarly to a chopped bar, melting a bit more than standard chips but still giving distinct pieces of chocolate throughout.
Does the cocoa version work for no-bake recipes? Not well — the melted butter and sugar mixture is designed to bake into the batter, not set up on its own the way real chocolate does once it cools, so no-bake recipes are better off with real chocolate.