Bring 1/4 inch water to a boil in a large skillet. Add the broccolini, cover with a lid, and cook for 8-10 minutes. Drain any remaining water then add the butter, soy sauce, grated ginger, sesame oil, and red pepper flakes to the pan and toss with the broccolini. Cover and cook for 2 minutes. Toss with the sesame seeds and season with kosher salt if needed.