In a dark, musty warehouse filled with row after row of aging cognac casks, Rebecca Jago, the joint managing director of Last Drop Distillers, is fidgeting with her phone’s flashlight. “Here,” she says, kneeling down and pointing underneath a 25-ton…

In a dark, musty warehouse filled with row after row of aging cognac casks, Rebecca Jago, the joint managing director of Last Drop Distillers, is fidgeting with her phone’s flashlight. “Here,” she says, kneeling down and pointing underneath a 25-ton barrel propped up on cinder blocks. “I think it was back there.”

I duck down to point my own torch into the darkness, only to see cobwebs and emptiness. But Jago, dressed in a red corduroy skirt and riding boots, sees something else: The spot where a wall once concealed a lonely vat of 1925 grand champagne cognac, which had been hastily hidden away from advancing German troops during World War II. That barrel of cognac was also the reason why Jago had taken me to the warehouse in the first place—Alexis Cabanne, the cellar master of this sixth-generation estate in southwest France’s Charente region, had sold the now rare, and very exclusive, spirit to her…