Combine the buttermilk, hot sauce, salt, garlic powder, onion powder, and smoked paprika in a large bowl and whisk until combined. Add the chicken, toss to coat, cover, and refrigerate for at least 2 hours, overnight is strongly preferred.
Crush the cornflakes by placing them in a zip lock bag and rolling firmly with a rolling pin until a coarse uneven crumble forms with some larger pieces still visible.
Set up your dredging station with three separate shallow bowls - the first dry dredge, the wet buttermilk and egg dip, and the thick cornflake crust.
Working one at a time remove each chicken plank from the brine letting the excess drip off. Press firmly into the first dry dredge until every surface is completely coated. Shake off the excess. Dip immediately into the buttermilk and egg mixture letting the excess drip off. Press firmly and aggressively into the thick cornflake crust making sure every surface is completely and thickly coated. Place on a wire rack and repeat with all remaining pieces.
Once all the chicken is dredged go back to the first piece and repeat the entire three-step dredge a second time. Return all double-dredged pieces to the wire rack and let rest 15 minutes before frying.
Heat neutral oil in a large deep skillet or Dutch oven to 300–325°F with at least 2 inches of oil.
Fry the chicken in batches of 2 without overcrowding for 3-5 minutes per side until the coating is pale golden and the chicken reaches 165°F internally. Remove and transfer to a wire rack and let rest for 5 minutes.
Increase the oil temperature to 375–400°F. Return the chicken in batches for 60–90 seconds per side until the coating is super crispy, golden, and craggy all over. Remove and drain immediately on a wire rack.
Make the spicy garlic parmesan sauce. Melt the butter in a small saucepan over medium-low heat. Add the minced garlic and cook for 1–2 minutes until fragrant and slightly golden - do not let it burn.
Remove from heat and whisk in the Maille Spicy or Original Mayonnaise, freshly grated Parmesan, heavy cream, lemon juice, black pepper, red pepper flakes if using, and a pinch of salt until completely smooth and combined.
Return to the lowest heat setting and stir until the Parmesan is completely melted and the sauce is silky and pourable. Taste and adjust salt and pepper as needed.
While the chicken is still hot transfer each piece into a large bowl and pour the warm garlic parmesan sauce generously over all the chicken. Carefully toss until every piece is completely and generously coated. Finish with extra freshly grated Parmesan and fresh chives over the top.
Serve immediately.