Prepare grill; set to medium heat.
Combine first 5 ingredients in large zip-top plastic bag. Seal bag; marinate in refrigerator 30 minutes, turning occasionally.
Remove duck from bag, reserving marinade. Sprinkle meat side of breasts with spice powder; place breasts on grill rack, skin side up. Cook 8 minutes on each side or until desired degree of doneness; remove from heat, and let stand 5 minutes.
Combine reserved marinade and broth in a small saucepan; bring to a boil. Cook until reduced to 1/4 cup (about 4 minutes).
Carefully remove skin from breasts; cut each breast in half. Cut each breast half into 1/2-inch-thick slices; arrange duck onto each plate. Spoon sauce over duck; sprinkle with cilantro, if desired.