Preheat the oven to 425 degrees F (220 degrees C). Line a baking sheet with foil
Peel sweet potatoes and cut crosswise into 3/4-inch-thick rounds.
Toss together sweet potatoes, oil, and gochugaru (Korean chile flakes) in a bowl. Spread out in a single layer on a foil-lined baking sheet.
Roast in the preheated oven until knife-tender, 15 to 18 minutes.
Meanwhile, bring brown sugar, soy sauce, orange juice and orange zest to a boil in a small saucepan over medium-high heat. Reduce heat and simmer, uncovered, until a thick, syrupy glaze forms, about 15 minutes. Remove potatoes from oven.
Position oven rack 6 inches from heat; preheat broiler. Lightly smash each potato slice with the bottom of a glass. Broil until browned in spots, about 3 minutes. Drizzle with glaze. Garnish with cilantro.