Put the sesame seeds in a large pot over medium-low heat and toast, stirring often, until golden and fragrant, 3 to 5 minutes. Transfer to a bowl. Fill the pot with at least 3 inches of oil and bring to 325°F over medium-high heat. Adjust the heat as necessary. If the oil starts to smoke, remove it from the heat immediately to cool a bit.
Put the gochujang, sugar, mirin, lemon juice, garlic, and ginger in a small saucepan and bring to a boil over medium heat. Reduce the heat so the mixture bubbles gently and cook, stirring occasionally, until the sugar has dissolved and the mixture has thickened to the consistency of barbeque sauce, 15 to 20 minutes. Taste and adjust the seasoning.
Put the cornstarch in a large bowl; sprinkle with salt and pepper. Add the wings and toss to coat.
When the oil is ready, shake off any excess cornstarch from the wings a few pieces at a time. Working in batches to avoid crowding the pan, carefully drop the wings into the oil. Cook, turning the pieces as necessary with a slotted spoon, for 5 minutes. Transfer the wings to a wire rack set over a rimmed baking sheet as they finish.
Bring the oil up to 375°F. Stir 2 tablespoons hot water into the gochujang sauce. When the oil is up to temperature, fry the wings again, until cooked through, crisp, and golden, 5 to 10 minutes. Transfer back to the rack and immediately brush with the sauce. If the sauce has thickened too much after cooling, stir in a tablespoon or so of water, but remember that the heat from the wings will help loosen it too. Sprinkle with the toasted sesame seeds and serve immediately, passing any extra sauce at the table.