Preheat the oven to 200 C / Gas 6. Line a baking tray with foil. Place a cooling rack on top.
Place cornflake crumbs in a bowl.
Mix teriyaki sauce, sriracha sauce, paprika, rice vinegar and sugar together in a large bowl until sauce has a thick consistency.
Place chicken in the bowl with the sauce; turn chicken to coat completely with sauce.
Place 1 piece of chicken into the bowl with the cornflakes. Spoon crumbs on top using a tablespoon. Press crumbs onto the chicken using the back of the spoon, pressing the chicken down into the crumbs to coat all sides. Place onto the baking rack, skin-side up. Repeat with remaining thighs and crumbs, placing largest pieces of chicken in the corner of the rack to receive the most heat.
Bake in the preheated oven for 30 to 40 minutes depending on the size of the chicken pieces. Reduce oven temperature to 160 C / Gas 3 and continue baking, 15 to 20 minutes. Turn chicken over to help crisp it up. Bake until cooked through, about 15 to 20 minutes more.
Let the chicken rest on the baking rack for at least 15 minutes before serving.
The sauce is not a marinade, so don't let the chicken thighs sit in it for very long, or else the salty ingredients will extract moisture that you want to remain in the thighs. Also, watery sauce won't hold the cornflake crumbs onto the chicken.