While the chicken and veggies are cooking, combine the hoisin sauce and the chicken stock in a small bowl. Add the liquid to the chicken along with the drained water chestnuts and stir to coat everything in the sauce. Cook for about another minute to heat up the water chestnuts, then add in the bean sprouts. Stir to combine and transfer to a serving bowl. Sprinkle the top with the toasted sesame seeds.
Remove the core of the iceberg lettuce by slamming the head, root end down, on the countertop -- really slam it! Once you turn the head over, you'll find that the root can be easily pulled out away from the leaves. Cut the head into 6 wedges and arrange the wedges around the bowl of chicken, making the leaves resemble the petals of a flower.
To assemble the wraps, grab a lettuce leaf and mound a spoonful of the chicken in the center. Wrap the leaf around the chicken and chomp away!