Whisk together soy sauce, rice vinegar, hoisin, brown sugar, broth, and sesame oil in a small bowl and set aside; arrange lettuce leaves on a platter and keep refrigerated until ready to fill.

Heat the 12-inch skillet over medium-high heat, add the ground beef and break it up; cook 6–7 minutes until browned and no pink remains, then spoon off and reserve 1 tablespoon fat and drain any excess if more than 2 tablespoons remain.

Return reserved fat to the skillet over medium heat, add ginger and garlic and cook 30 seconds until fragrant, then add shredded carrots and diced water chestnuts and cook 2–3 minutes until vegetables are tender-crisp.

Return beef to the skillet, pour in the prepared sauce, bring to a simmer over medium, then reduce heat to medium-low and cook 3–5 minutes until sauce thickens and becomes glossy, stirring so it coats the beef; stir in half the sliced scallions and remove from heat.

If mixture seems too thin, stir in cornstarch slurry and simmer 1 more minute until thickened.

Spoon 2–3 tablespoons filling into each lettuce leaf, garnish with remaining scallions and sesame seeds, and serve immediately.