In a resealable plastic bad, mix buttermilk, dijon mustard, 1 tablespoon onion powder, 1 teaspoon salt, 1 teaspoon dry mustard, 1 teaspoon cayenne, and 1 teaspoon black pepper. Add chicken pieces. Seal bag, eliminating air. Turn to coat chicken evenly. Refrigerate at least 1 day (up to 2), turning occasionally.
Whisk flour, baking powder, garlic powder, and remaining 1 tablespoon onion powder, 4 teaspoon salt, 3 teaspoon dry mustard, 3 teaspoon cayenne, and 1½ teaspoon black pepper in13 x 9 x 2 inch baking dish. With marinade still on chicken pieces (do not shake off excess), add chicken to flour mixture, turning to coat thickly. Let stand in flour (1 hour) turning occasionally to re-coat.
Pour enough oil to reach depth of 1¼ inch into a wide, deep pot. Heat oil over medium-high heat to 350 ̊F. Add 4 pieces of chicken, skin side down, to oil. Reduce heat to medium-low and fry (5 minutes). Adjust heat to maintain oil temperature between 280 ̊F and 300 ̊F (should bubble constantly around chicken). Using a wooden spoon, turn chicken. Fry (7minutes). Turn over again. Fry until deep gold brown and cooked through (3 minutes). Transfer chick to a large rack set over a baking sheet.
Reheat oil to 350 ̊F. Repeat frying with remaining chicken.