There are several elements involved in cognac production to make it the finest of all spirits;

Eaux-de-vie specifically refers to the grape brandies obtained from the Double distillation of white wine made from specific grape varieties grown in the Cognac region of France.  By subjecting the eaux-de-vie to double distillation, the impurities are removed and concentrated to the desirable characteristics of the grape spirit, resulting in a smoother more refined base for aging and blending.  The double distillation process is a hallmark of cognac production, contributing to the spirits elegance and complexity. 

These eaux-de-vie are then ages in oak barrels to develop their unique flavors and aromas before being blended to create cognac.