In a large bowl, combine buttermilk and hot sauce. Add chicken tenderloins, turn to coat, cover, and refrigerate for at least 30 minutes (or up to 4 hours).
In a shallow dish, whisk together flour, cornstarch, seasoning salt, garlic powder, paprika, black pepper, and cayenne. In a second shallow dish, beat eggs with 2 tablespoons water or buttermilk.
Remove a tenderloin from buttermilk, letting excess drip off. Dredge in seasoned flour, pressing gently to coat. Dip into egg wash, then back into the flour mixture, pressing firmly to create a thick crust. Place on a wire rack.
Heat oil in a large, heavy skillet or Dutch oven to 350°F (175°C). Carefully add tenders in a single layer without crowding. Fry 3-4 minutes per side, until deep golden brown and internal temperature reaches 165°F (74°C). Drain on paper towels.
Immediately sprinkle hot tenders with a pinch of flaky sea salt. Serve with honey mustard, ranch, or barbecue sauce.