Cut the chicken into pieces if it isn't already, and score the thighs once through the skin so the marinade can reach the meat. Toss with the buttermilk in a bowl, cover, and refrigerate at least 1 hour or overnight.

Drain the chicken in a colander, shaking off the excess buttermilk but not patting it fully dry — a damp surface helps the starch cling.

Season the chicken with the salt and pepper, then dredge each piece in the potato starch, pressing it on firmly. Let the coated pieces rest on a wire rack for 10 minutes; the starch turns slightly translucent and sticky, which is what you want.

Heat the oil in a deep, heavy pot to 160°C / 320°F. Fry the chicken in batches for 8 to 9 minutes, turning once, until pale gold and just cooked through. Don't crowd the pot — the oil temperature drops fast if you do. Drain on a rack.

Let the chicken rest 5 minutes, then raise the oil to 190°C / 375°F. Fry each batch again for 2 to 3 minutes, until deep golden brown and audibly crackling when tapped with tongs. This second fry is what makes the crust hold up under a wet glaze.

While the chicken rests between fries, combine the soy sauce, brown sugar (and grated pear if using), rice wine, garlic, ginger, and gochujang in a small saucepan. Bring to a simmer over medium heat and cook 3 to 4 minutes, stirring, until it thickens enough to coat the back of a spoon.

Turn the heat to low and stir in the 2 tablespoons of water to loosen the glaze slightly — it should coat a spoon and drip slowly, not sit like syrup.

Add the hot, double-fried chicken straight to the saucepan and toss with a spatula until every piece is coated and glossy, about 30 seconds. Work quickly; the glaze tightens as it cools and gets harder to spread evenly.

Transfer to a platter, drizzle with the sesame oil, and scatter the sesame seeds over the top. Serve immediately — the crust is at its best in the first 15 minutes.