Using ground dark meat (chicken thighs) produces a much juicier meatball compared to using leaner ground chicken breast. The breadcrumbs and egg help the meatballs retain their shape and cling to the skewer throughout the grilling process. Also, they're less likely to fall apart on the grill if you let them get well browned on the first side before flipping to grill the other side. And a final brushing of the Tare Sauce will coat the slightly spicy meatballs in a sweet and salty glaze.
1For the Tare Sauce: Combine mirin, soy sauce, sake, brown sugar, sherry vinegar, garlic, scallions, ginger slices, and peppercorns in a medium saucepan. Bring to boil over high heat, then reduce to a simmer, whisk to combine, and cook until mixture is thick and syrupy, about 45 minutes. Strain through a fine mesh strainer. Set aside or store in an airtight container in the refrigerator until ready to use.
While preparing meatballs, soak wooden skewers in water for 30 minutes (to prevent charring during grilling).
2For the Meatballs: Using hands, mix together chicken, bread crumbs, scallions, egg, ginger, garlic, sesame oil, salt, and white pepper until thoroughly combined. Form mixture into 1-inch meatballs and thread onto skewers.
3Heat grill to medium-high heat. Oil the grilling grate. Careful transfer skewers to grill, if necessary supporting meatballs from bottom to prevent them from falling off skewers. Grill until first side is well browned, about 3-4 minutes. Using tongs, rotate meatballs and cook until well browned on second side, about 2-3 minutes. Repeat for remaining two sides.
4Brush tare sauce all over meatballs and allow to cook for 15-30 seconds longer. Transfer skewers to a plate or serving dish, brush lightly with sauce again and let rest for 5 minutes. Serve immediately.