In medium bowl, whisk buttermilk, mustard powder, Sriracha, garlic, and paprika. Place chicken in large zip-top bag; pour buttermilk mixture over chicken. Seal bag and refrigerate 2 hours or overnight.
Heat outdoor grill for direct grilling over medium heat. Remove chicken from marinade, shaking off excess; discard marinade. Lightly oil grill grates. Transfer chicken to grill and cook, turning occasionally, 16-18 minutes, or until internal temperature reaches 165 F°. Transfer chicken to serving platter. Sprinkle with parsley and serve with lemon wedges, if desired.