For the spicy mayo: place all ingredients in a small bowl and mix until combined. Refrigerate until ready to use.
For the oven fried chicken: In a large bowl, pour the buttermilk and season with salt and pepper.
Place the chicken in the buttermilk and mix until evenly coated. Cover and place in the fridge for 1-2 hours or overnight.
Preheat oven to 180 C /365 F and arrange a baking tray in the middle shelf.
Place cornflakes, panko, olive oil, spices (if using) in a food processor, and season with salt and pepper to taste. Mix until finely crumbled and place in a large bowl.
Shake off the excess buttermilk from the chicken, then dip each piece in the cornflakes mixture and coat evenly, patting with fingers to make the crumbs stick.
Cover the hot baking tray with parchment paper, spray with a little olive oil and arrange the chicken tenders over it, leaving a bit of space for the buns.
Bake for 20 minutes, or until the chicken internal temperature reaches 73°C / 165°F.
Over the last 5 minutes of baking time, pop the halved buns into the baking tray together with the chicken, and bake until crisp.
To assemble: Divide the chicken tenders and vegetable toppings among the 4 burgers, top with spicy mayo and serve immediately.