Whisk soy sauce, rice vinegar, sesame oil, brown sugar, garlic, ginger, red pepper, and black pepper in a bowl.
Add steak cubes and toss to coat. Marinate 15 minutes, then blot the steak dry with paper towels.
Stir cornstarch and water in a small cup to make a smooth slurry; set aside.
Heat a large skillet over high heat. Add vegetable oil and swirl until shimmering.
Sear steak in two batches, 1–2 minutes per side, until browned outside and medium inside. Transfer to a plate.
Lower heat to medium. Add butter to the skillet and melt.
Pour any leftover marinade from the bowl into the pan and bring to a full boil. Stir in the cornstarch slurry and simmer until glossy, 30–60 seconds.
Return steak and any juices to the pan. Toss to coat in the sauce, then finish with green onions and sesame seeds.