Whisk together the hoisin, soy sauce, vinegar, sriracha, and cornstarch. 

Set aside until ready to use. If the cornstarch settles to the bottom, whisk again before using. 

Add the sesame oil to a large skillet set over medium heat. 

When the sesame oil is fragrant, add the ground chicken and saute, breaking up the chicken, until it is cooked through and no longer pink, about 5-7 minutes. 

Then, add the onion, garlic, and ginger. Saute for an additional 3-4 minutes or until the onions are soft. 

Add the sauce to the pan and stir until the chicken is evenly coated. Cook for another minute. If the sauce thickens too quickly, thin it with ¼ cup water. 

Next, add the water chestnuts and green onions. Toss to combine and remove from the heat. 

Add a spoonful of the chicken mixture to a piece of butter lettuce and serve. 

Store any leftover filling in an airtight container in the fridge for up to three days. Reheat and then fill the lettuce leaves as desired.