For the Doenjang Mayo: Mix the mayonnaise and doenjang together well with a spoon in a small bowl. Cover and place in the fridge.
For the Marinade and the Chicken:
In a large bowl, whisk together the soy sauce, brown sugar, maple syrup, vinegar, chilli paste, ginger, sesame oil, sesame seeds, black pepper, salt, garlic and spring onions. Reserve about 1 cup (235 milliliters) to make the glaze.
Place the chicken thighs in a medium bowl and pour in the remaining marinade, making sure all sides are covered. Cover and place in the fridge for at least 4 hours and better yet, overnight, to let the flavors infuse.
Preheat the oven to 230°C. Bring the chicken up to room temperature for 30 minutes to 1 hour before grilling.
Wrap the sweet potatoes in a sheet of foil, sealing well. Bake until a small knife goes all the way through the potatoes easily, about 1 hour. Let cool slightly, then peel the skins off and discard. Place the potatoes in a small bowl and mash roughly with a fork. Cover and set aside in a warm place.
Place a saucepan over medium-low heat and tip in the reserved marinade to make a glaze to brush over the chicken. Simmer until thickened, 8 to 10 minutes.
In the meantime, bring a grill or griddle pan to medium-high heat. Brush lightly with vegetable oil. Shake the excess marinade off the chicken and place on the grill skin-side down first. Cook until cooked through, flipping once, about 15 minutes. Let rest in a warm place while you cook the corn.
Place the corn on the grill and cook, turning often, 9 to 10 minutes. Cut each piece down the center lengthwise.
To serve, transfer the chicken to a serving plate or platter. Brush the chicken very lightly with the glaze, or serve it on the side as a dipping sauce. Garnish the chicken with the chilli threads. Finish the sweet potatoes off with a sprinkle of black sesame seeds. Serve the corn with the doenjang mayo on the side or brush it on to the hot corn, letting it melt into the crevices.