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.

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.

Light one chimney full of charcoal. When all charcoal is lit and covered with gray ash, pour out and spread coals evenly over entire surface of charcoal grate. Set cooking grate in place, cover grill and allow to preheat for 5 minutes.

Clean and oil grilling grate. Carefully 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.

Brush 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.