Whisk the buttermilk, hot sauce, and 1 teaspoon salt in a large bowl. Add the thighs, cover, and refrigerate for at least 4 hours. Overnight works even better.
Combine flour, cornstarch, paprika, garlic powder, onion powder, black pepper, cayenne, and remaining salt in a shallow dish.
Pull each thigh from the brine and press it firmly into the flour mixture. Set the coated pieces on a wire rack for 10 minutes so the crust sticks.
Pour the oil into a heavy pot and bring it to 350°F. Use a thermometer.
Cook 3 to 4 pieces at a time for 6 to 8 minutes, flipping once. Chicken is done at 165°F inside. Rest the pieces on a clean wire rack, never paper towels.
Whisk the dry ingredients, then stir in the buttermilk, egg yolks, melted butter, and vanilla. Beat the egg whites to soft peaks and fold them in gently.
Bake in a hot, greased waffle iron until deep golden, about 4 to 5 minutes each.
Stack waffle first, then butter, then chicken, then maple syrup. Serve immediately.