Peel the potatoes and cut them into 1/4-inch shoestrings using a mandate or sharp knife.

Soak the cut potatoes in a large bowl of cold water for at least 1 hour to remove surface starch.

In a separate bowl, mix the hot water, corn syrup (or sugar), and vinegar until dissolved. Drain the cold water from the potatoes and submerge them in this brine for 15 minutes.

Remove potatoes from the brine and dry them thoroughly with paper towels. Moisture is the enemy of crispiness.

Heat the beef tallow (or oil) in a deep fryer or Dutch oven to 300°F (150°C).

Working in batches, par-fry the potatoes for 60 to 90 seconds. They should be soft and limp but strictly not browned.

Remove fries and drain on a wire rack. Let them cool, then arrange in a single layer on a baking sheet and freeze for at least 2 hours (or overnight).

Heat the fat to 400°F (205°C). Fry the frozen potatoes in batches for 3 to 4 minutes, or until golden brown and crispy.

Transfer immediately to a bowl lined with paper towels and toss with fine sea salt while hot.