Place the cubed chicken and the buttermilk in a gallon-size zip-top bag, seal, and toss to coat. Marinate briefly while you prepare the other components, at least 10 minutes if time allows.
Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper and set aside.
In a medium bowl, combine the finely crushed corn flakes, flour, and a pinch of black pepper; mix well.
Working with one piece at a time, remove chicken from the buttermilk and dredge in the corn flake mixture, pressing gently so the crumbs adhere. Arrange the coated pieces on the prepared baking sheet in a single layer without crowding; use a second sheet if needed.
Lightly brush the tops of the coated chicken pieces with extra virgin olive oil. Bake for 15–20 minutes, until the chicken is cooked through and the coating is golden and crisp.
While the chicken bakes, make the Korean BBQ sauce: in a small saucepan combine the soy sauce, honey, gochujang, rice vinegar, grated ginger, grated garlic, toasted sesame oil, and 1/4 cup water. Bring to a boil over high heat, then reduce to a simmer and cook 5–10 minutes until slightly thickened into a glaze. Remove from heat.
When the chicken is done, brush or drizzle the pieces generously with the Korean BBQ sauce. Sprinkle with toasted sesame seeds and chopped green onions, then serve warm.