Marinate chicken. Add the chicken pieces to a bowl and pour in the buttermilk along with salt, black pepper, and garlic powder. Stir to coat evenly, cover, and marinate for at least 30 minutes or overnight for best flavor.
Prepare dredge. In a separate bowl, whisk flour and cornstarch with paprika, onion powder, cayenne if using, salt, and pepper. Remove the chicken from the marinade and coat each piece thoroughly, pressing the flour mixture onto the surface to create a textured crust.
Heat oil. Bring neutral oil in a deep pot or skillet to 350°F (175°C).
Fry chicken. Fry the coated chicken in batches without overcrowding until deeply golden and crispy, about 5 to 7 minutes depending on size. Transfer to a wire rack to drain.
Make sauce. In a saucepan, combine passionfruit pulp with brown sugar, honey, soy sauce, minced garlic, grated ginger, rice vinegar, and ketchup. Bring to a gentle simmer and cook for 3 to 5 minutes while stirring occasionally. Stir in the cornstarch slurry and cook until the sauce becomes glossy and sticky, then whisk in butter until smooth.
Toss chicken. Add the hot fried chicken to the sauce and gently toss until evenly coated.
Garnish and serve. Transfer to a serving dish and finish with sesame seeds, sliced green onions, lime zest, or chili flakes if desired. Serve immediately while crispy.