Prepare the Chicken: In a bowl, combine flour, cornstarch, salt, and pepper. In another bowl, whisk the egg. Dip each chicken piece first into the egg, then dredge in the flour mixture, ensuring it's fully coated.

Fry the Chicken: Heat vegetable oil in a large skillet or deep pot over medium-high heat to 350°F (175°C). Carefully add chicken pieces in batches, making sure not to overcrowd the pan. Fry for 5-7 minutes per batch, or until golden brown and cooked through. Remove with a slotted spoon and place on a wire rack set over paper towels to drain excess oil.

Make the Gochujang Glaze: In a small saucepan, combine 1/4 cup gochujang, soy sauce, honey (or brown sugar), 1 tablespoon rice vinegar, minced garlic, grated ginger, and 1 tablespoon sesame oil. Bring to a simmer over medium heat, stirring occasionally, until the glaze thickens slightly, about 3-5 minutes.

Toss the Chicken in Glaze: Add the fried chicken pieces to the saucepan with the glaze. Toss gently to ensure all chicken pieces are evenly coated.

Prepare the Gochujang Mayo: In a small bowl, whisk together mayonnaise, 1 tablespoon gochujang, 1/2 teaspoon sesame oil, and 1/2 teaspoon rice vinegar until smooth.

Serve: Plate the spicy gochujang glazed chicken immediately. Serve with a generous side of the homemade Gochujang mayo for dipping, and garnish with sesame seeds or chopped green onions if desired.