Make the spicy mayo by combining the mayonnaise, sriracha, grated garlic and finely chopped pickles (if using) in a small bowl; mix until smooth, then refrigerate until ready to use.
Place the buttermilk in a large bowl and season with sea salt and black pepper; add the chicken pieces and turn to coat. Cover and refrigerate for 1–2 hours, or overnight if time allows.
Preheat the oven to 180°C (365°F). Line a baking tray with parchment paper and set it on the middle rack.
Pulse the cornflakes, panko, olive oil and optional spices (cayenne and ground garlic) in a food processor until they form fine crumbs; season the crumb mixture with salt and pepper to taste and transfer to a large bowl.
Remove each chicken piece from the buttermilk, letting excess drip off, then press into the crumb mixture to coat evenly, patting so the crumbs stick.
Arrange the coated chicken on the prepared tray with some space between pieces; lightly brush or spray the top with a little olive oil.
Bake for about 20 minutes, or until the chicken reaches an internal temperature of 73°C (165°F) and the coating is golden and crisp.
During the last 5 minutes of baking, place the halved buns cut-side up on the tray to toast until crisp.
To assemble each burger, layer lettuce, a slice of tomato and red onion on the bottom bun, add a chicken tender, spread spicy mayo on the top bun, then close and serve immediately.