For the hoisin duck sauce: Combine the orange juice, hoisin sauce, light soy sauce, brown sugar, chili-garlic paste, garlic, Sichuan pepper and white pepper in a saucepan or wok over high heat. Cook until slightly thickened, about 10 minutes.
For the crispy popcorn duck: Combine the cubed duck and soy sauce in a small bowl. Add the crushed pretzels, potato starch, five-spice powder, dried chili flakes and black pepper, and toss to evenly coat.
Heat a wok over medium heat and add enough oil to deep-fry the duck. When the oil reaches 350 to 375 degrees F, quickly fry the duck until crispy, 45 seconds to 1 minute. Transfer the duck to drain on a paper-towel-lined plate and keep warm.
For the vegetable stir-fry: Remove all the oil from the wok except for 1 to 2 tablespoons and heat over a high flame. Quickly stir-fry the cashews, red bell pepper, scallions and water chestnuts for 30 seconds. Add the crispy duck back to the wok to heat through. Transfer to a plate and keep warm.
To serve: Wrap the crispy duck stir-fry in the lettuce cups and top with the scallion strips. Serve the hoisin duck sauce on the side.