Heat the oven to 425°F and set a greased wire rack over a rimmed sheet pan.
Whisk the buttermilk, mustard and salt in a shallow bowl.
Combine the coarse cornflake crumbs, paprika, garlic powder and pepper in another pan and drizzle in 1 tablespoon oil while tossing.
Dip each thigh in the buttermilk mixture and press it hard into the cornflakes on both sides.
Arrange the thighs on the rack with space between them and brush or spray the remaining oil lightly over dry crumb patches.
Bake for 30 to 35 minutes without turning until the coating is deep gold and the thickest thigh reaches at least 165°F.
Rest on the rack for 6 minutes so the bottom stays crisp.