Cut the Pork Shoulder (Butt) into 1–1½ inch bite-sized cubes and pat dry with paper towels.
Make a marinade by whisking together the Egg White, Shaoxing Wine, Light Soy Sauce, a pinch of Fine Salt and White Pepper, and 1 tablespoon Cornstarch; add the pork, mix to coat, and refrigerate for 20–30 minutes.
Prepare the batter by combining ½ cup Plain Flour, 2–3 tablespoons more Cornstarch, 1 teaspoon Baking Powder, and enough Water to make a thick, pancake-batter consistency; dip the marinated pork into the batter to coat.
Heat enough Neutral Oil in a deep pan or wok to 325°F (moderate) and fry the battered pork in batches until set and just turning light golden (about 3–4 minutes); drain on a rack.
Increase the oil temperature to 375°F (hot) and return the pork in batches for a second fry until deep golden and very crisp (1–2 minutes); drain and keep warm.
For the sauce, whisk together ½ cup Ketchup, 3 tablespoons Rice Vinegar, 3 tablespoons Sugar, 1 tablespoon Light Soy Sauce, and ¼ cup Pineapple Juice in a bowl.
Heat a tablespoon of Neutral Oil in a wok over high heat, stir-fry the Red Bell Pepper, Green Bell Pepper and Onion until just tender-crisp (1–2 minutes), then add the Pineapple Chunks and briefly toss.
Pour the sauce into the wok and bring to a simmer. Mix 1 tablespoon Cornstarch with 2 tablespoons Water to form a slurry, stir it into the sauce to thicken to a glossy coating.
Add the double-fried pork to the wok and toss quickly to coat everything evenly in the sauce. Serve immediately over steamed rice.
