70% refers to the percentage of cocoa solids, cocoa butter and cacao mass with the remaining 30% of sugar and other ingredients in the chocolate.
Cacao beans are the main ingredient for dark chocolate from which cocoa butter and cacao mass is extracted. It is then mixed with sugar for sweetening.
Cacao beans are harvested from the fruits of cacao tree, after which the beans are fermented, dried and roasted to develop rich flavor.
The roasted cacao beans are grounded to a paste called cacao mass which is then refined and conched to smoothen the texture and make flavor intense.
Final step is to bring a glossy finish to the chocolate by tempering it and its is then poured to mold and frozen.