Step 1
To a zip-top bag, add the honey, mirin, sake, soy sauce, ginger, duck hearts, and garlic. Seal and gently massage to coat. Refrigerate for at least 1 hour, or up to 12.
Step 2
Soak four bamboo skewers in water for 30 minutes. Preheat a grill to high, or position a rack just below the broiler and preheat to high. Thread the hearts evenly onto the skewers, discarding the marinade. Cook, turning every 2 minutes, until the hearts are lightly charred, firm to the touch, and light pink inside when cut open, 3–6 minutes. Sprinkle with sesame seeds and serve with pickled watermelon rind on the side.