Marinate the chicken - In a medium mixing bowl whisk together the regular soy sauce, dark soy sauce, oil, and cornstarch. Add the chopped chicken to the bowl and thoroughly mix to coat the chicken in the marinade. Set aside.
Prepare the sauce - As the chicken marinates, combine the oyster sauce, sugar, regular soy sauce, dark soy sauce, and fish sauce. Stir well to combine.
Prepare rice noodles - Remove rice noodles from the refrigerator and allow them to come to room temperature. If they come folded in uncut sheets, slice them into ribbons approximately 3/4-inch wide. Place the folded up rice noodles in a large colander and rinse under hot water to help separate the stuck-together ribbons. Rinse off any additional water. If using dried rice noodles, cook according to package instructions, making sure to undercook slightly since they will continue to cook when they are stir-fried with the rest of the ingredients.
Cook the chicken - Add 1 tablespoon oil to a large wok or heavy-bottomed skillet set over high heat. Once hot, add the chicken and cook for 5-6 minutes, mixing frequently, to ensure even cooking. Once cooked, remove to a clean plate and set aside.
Sauté the aromatics and Chinese broccoli - Return the wok or skillet to high heat. Add an additional tablespoon of oil and add the minced garlic and chopped Thai chilis (if using). Immediately add the Chinese broccoli and cook, stirring continuously for 1-2 minutes. Remove to a clean plate and set aside.
Add the noodles and sauce - Return the wok or skillet to high heat. Add an additional tablespoon of oil and, working quickly, add the noodles to your pan immediately followed by the sauce. Gently mix to combine. Continue to cook for approximately 20-30 seconds before returning the chicken and vegetables back to the pan.
Fry the egg - Push the noodles and chicken to one side of the skillet. Add one more tablespoon of oil to the empty side of the skillet and add the eggs. Wait 5-10 seconds for the eggs to start cooking before scrambling and breaking into smaller pieces.
Finish cooking and serve - Allow the stir fry to cook until the noodles are evenly heated through (approximately 1-2 minutes). Be careful not to over-mix or the noodles will start to break apart. Some noodles will stick, but with the heat high enough, they should start to stick less. When in doubt, add a teaspoon or two of additional oil as needed. Best served immediately.