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 chicken: In a large bowl, whisk together the soy sauce, brown sugar, maple syrup, vinegar, chile 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 450 degrees F (230 degrees 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 chile 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.
Make sure your grill isn't too hot, as your chicken will burn on the outside before the insides are cooked all the way through. You may need to adjust your grill between medium and medium-high heat. Have your butcher debone the thighs for you, or simply use bone-in thighs--just adjust the cooking time accordingly. This mayo is great for dipping the chicken in as well as smearing on the corn. You'll want to make it to eat with French fries too.