In a small bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, and sriracha until the sugar dissolves. Set aside.

In a separate cup, mix the cornstarch with cold water to create a slurry. This will be used to thicken the sauce later.

Heat a large skillet over medium-high heat and add the sesame oil. Add the ground chicken, breaking it up with a wooden spoon. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat if necessary.

Reduce the heat to medium. Add the diced onion and cook until softened, about 3 minutes. Stir in the minced garlic and grated ginger, cooking for 1 minute more until fragrant.

Add the chopped water chestnuts (if using) and stir to combine. Pour in the prepared teriyaki sauce and bring to a simmer, stirring constantly.

Give the cornstarch slurry a quick whisk and pour it into the skillet. Cook for 1-2 minutes, stirring, until the sauce thickens into a glossy glaze that clings to the chicken. Remove from heat and stir in half of the sliced green onions.

Arrange the butter lettuce leaves on a serving platter. Spoon the warm teriyaki chicken into each lettuce cup. Top with sesame seeds, remaining green onions, and any additional toppings like shredded carrots, chopped nuts, or cilantro. Serve with lime wedges on the side for a fresh finish.