For the marinade: In a large bowl, mix the buttermilk, sugar, salt, and hot sauce until combined. Add the chicken and toss to coat. Cover and refrigerate for at least 4 hours and up to 24 hours, turning occasionally.
Remove the chicken from the buttermilk marinade and pat completely dry. Place the chicken on a wire rack set over a rimmed baking sheet and let sit at room temperature for 30 minutes before frying.
For the batter: In a large bowl, whisk together the flour, cornstarch, baking powder, salt, pepper, paprika, and chile de arbol until combined. Slowly whisk in the water and whisk until smooth. Let the batter sit at room temperature for 15 minutes.
Heat the oil to 350 degrees F in a large Dutch oven. Transfer half the chicken into the batter. Remove the chicken, 1 piece at a time, from the batter (allowing any excess to drip back into the bowl) and carefully transfer to the oil. Fry the chicken, adjusting the burner as necessary to maintain the oil temperature between 300 and 325 degrees F, until deep golden brown and just cooked through, about 12 minutes. Drain the chicken on paper towels and then transfer to a wire rack set over a rimmed baking sheet and season with a little fine sea salt. Bring the oil back to 350 degrees F and repeat with the remaining chicken.