Slice the sirloin steak into thin strips against the grain. Toss the steak with cornstarch until each piece is well coated.
Heat the vegetable oil in a large skillet over medium-high heat. Add the beef strips to the hot oil and cook the beef in batches so the pan is not overcrowded. This will give the beef a nice crispy texture. Overcrowding the pan will cause the meat to steam and will not give it the crispy, velvet texture. Cook the beef for about 10 minutes until it's browned and cooked through.
Add the chopped broccoli florets and the white parts of the scallion to the pan and cook for a few minutes until soft.
Prepare the ramen noodles per package directions.
Prepare the sauce by whisking together in a large measuring cup or medium bowl the soy sauce, brown sugar, ground ginger, minced garlic, red pepper flakes, sriracha and the chicken broth.
Remove the steak, broccoli and onions from the pan and add the sauce. Be sure to scrape up all the brown bits from the bottom of the skillet as that is all delicious flavor. Cook the sauce for a few minutes until it thickens and reduces. Add the steak, broccoli and noodles to the skillet and stir to evenly coat with the sauce.
Top with the green scallions and serve with extra sriracha on the side (if desired).