In a large bowl, combine the thinly sliced flank steak with 1 tbsp soy sauce, 1 tbsp Shaoxing wine, 1 tsp cornstarch, 1 tsp sesame oil, and 1/4 tsp baking soda (if using). Mix well to coat the beef evenly. Let marinate at room temperature for at least 15 minutes, or refrigerate for up to 30 minutes.

In a small bowl, whisk together 2 tbsp soy sauce, 1 tbsp dark soy sauce, 1 tbsp black vinegar, 1 tbsp sugar, 1 tbsp cornstarch, 1/2 cup chicken broth, and 1 tsp sesame oil until smooth. Set aside.

Heat 1 tbsp of vegetable oil in a large wok or skillet over high heat until smoking. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding) and stir-fry for 1-2 minutes until browned and mostly cooked through. Do not overcook. Remove the beef from the wok and set aside.

Add the remaining 1 tbsp of vegetable oil to the wok. Reduce heat to medium-high. Add the dried red chilies and Szechuan peppercorns (if using). Stir-fry for 30-60 seconds until fragrant and the chilies have darkened slightly (be careful not to burn them).

Add the minced garlic, grated ginger, and the white parts of the sliced green onions to the wok. Stir-fry for another 30 seconds until fragrant.

Return the seared beef to the wok. Give the prepared Kung Pao sauce a quick re-whisk and pour it into the wok. Stir quickly to coat everything evenly. The sauce will thicken rapidly.

Add the roasted peanuts and the green parts of the sliced green onions. Toss for another 30 seconds until everything is well combined and heated through.

Remove from heat and serve immediately with steamed white rice.