Can I use a 100% cocoa bar melted instead of cocoa?

To substitute broken chocolate or chips for cocoa, what's the appropriate measure?

Well...… the bar will contain cocoa butter; the usual suggestion for substitution of cocoa for chocolate is 3 Tb. cocoa and 1 Tb. oil for 1 oz. unsweetened chocolate; you could work back from that to get a quantity. The cocoa butter will effect the recipe to some extent- you could either account for it by reducing other fat in the recipe, or let it go and hope for the best; it's very unlikely to be disastrous, you might even prefer it.