
Chocolate cordial is a bar preparation rather than a bottle: cacao nibs or cocoa steeped, sweetened and acidified, so it brings sourness along with the chocolate and the sugar.
There is little to buy, because the cordial is usually made in-house: steep cacao nibs in warm sugar syrup, strain, and finish with citric acid or lemon. The acid is not a detail but half the point, and it is what every shop-bought stand-in leaves out.
Cocoa and sugar in water, with nothing sour in it. It sweetens and does not sharpen, so in place of the cordial the drink loses its lift and needs a squeeze of citrus put back.
Chocolate liqueur
A liqueur such as crème de cacao is an alcoholic sweetened distillate, where a cordial in the sweetener sense is a non-alcoholic concentrate. In American bar usage "cordial" doubles as a word for liqueur, which is where the confusion starts — and pouring one for the other changes both the strength and the sugar of the drink.
Chocolate cordial confection
The confection is a solid chocolate-shelled sweet with a liquid center, not a pourable syrup or drink ingredient; searches for "chocolate cordial" most often return this cherry-cordial style candy.
Homemade keeps a couple of weeks chilled — the acid buys it some of that time — and cocoa settles out as it stands, so it wants shaking before it is measured.