In a large mixing bowl add the buttermilk, Dijon mustard, 1 tablespoon onion powder, 1 teaspoon salt, 1 teaspoon dry mustard, 2 teaspoon dry spice rub, 1 teaspoon black pepper and whisk together. Add chicken pieces. Refrigerate at least 4 hours.
Whisk flour, baking powder, garlic powder, remaining 1 tablespoon onion powder, 3 teaspoons salt, 3 teaspoons dry mustard, 2 teaspoons dry spice rub and 1 teaspoon black pepper in a large mixing bowl. With marinade still clinging to chicken pieces (do not shake off excess), add chicken to flour mixture. Toss the chicken and coat with flour mixture.
Pour oil to depth of 1 1/4 inches into deep pot. Heat oil over medium-high heat to 350°F. Add 4 pieces of chicken, skinned side down into the oil. Reduce heat to medium-low and fry 5 minutes, adjusting heat to maintain oil temperature between 280°F and 300°F. Turn chicken over and continue to fry for 7 minutes. Turn chicken over again. Fry until deep golden brown and cooked through, about 3 minutes longer. Remove the chicken from the oil and place onto paper towel to dry.
Reheat oil to 350°F. Repeat frying with remaining 4 pieces of chicken. Serve chicken warm or at room temperature within 2 hours, or chill up to 1 day and serve cold.