4 (6-ounce) boneless, skinless chicken breasts
2 tablespoons dark brown sugar
2 teaspoons ground coriander
1 1/2 teaspoons kosher salt
1 1/2 teaspoons dry mustard powder
1/4 teaspoon cayenne
1 1/2 tablespoons extra-virgin olive oil, more as needed
4 tablespoons Dijon mustard
2 teaspoons minced fresh chives
Place chicken breasts between two sheets of parchment or plastic wrap. Using a mallet or rolling pin, pound each to an even thickness of 1/2 inch. Do not make them any thinner or they could dry out.
In a small bowl, combine sugar, coriander, salt, mustard powder and cayenne. Place chicken breasts in a bowl and rub well with spice mixture. Cover and refrigerate 1 to 2 hours.
Remove chicken from fridge while you heat the grill.
Light the grill, building a hot fire, or heat your gas grill to high. Once grill is fully heated, brush breasts lightly with olive oil and place chicken on the grill. Cook until undersides are browned and chicken is about halfway cooked, 3 to 5 minutes.
Flip breasts and grill until cooked through, 3 to 5 minutes more.
Transfer chicken to a platter.
In a small bowl, whisk together mustard and chives. Whisk in 1 1/2 tablespoons oil. Serve chicken with mustard for dipping.