Working in batches, cook the bacon in a large skillet over medium heat until crisp; drain on paper towels. Sprinkle the bacon with the five-spice powder, then break the slices in half.
Preheat the broiler. Place the tomato slices on a baking sheet and brush with half of the hoisin sauce. Broil until bubbling, about 3 minutes, then turn; brush with the remaining hoisin sauce and broil 3 more minutes.
Combine the mayonnaise and Sriracha in a small bowl. Toss the watercress and cilantro in a medium bowl with the vinegar, sesame oil and a pinch of salt.
Dampen 2 paper towels and squeeze out the excess water. Wrap the Chinese buns loosely in the paper towels, then place in a large microwave-safe bowl; cover with plastic wrap and microwave in 30-second intervals until hot and soft.
(Alternatively, steam in a steamer basket set over boiling water, 5 minutes.) If using hamburger buns, do not steam.
Spread the inside of the buns with some of the chile mayonnaise. Fill the buns with a few slices each of cucumber, bacon and tomato, and some of the watercress mixture.