Soak the bamboo or wooden skewers in cold water and refrigerate for 1 hour.
Cut the steak into 24 (3 by 1-inch) strips, each weighing about 1/2-ounce. Stick a skewer into each strip lengthwise and arrange on a large platter or baking pan. Refrigerate until needed.
Prepare the marinade: In a small bowl, combine all the marinade ingredients and pour over the meat, turning to coat all sides. Let marinate at room temperature for about 15 minutes.
Preheat the grill or broiler while the steak is marinating.
Prepare the sauce: In a small skillet, melt 2 tablespoons butter. Add the garlic and green onion and saute over medium-high heat until soft, about 2 minutes. Pour in the hoisin sauce, soy sauce and chili pepper flakes and cook 1 or 2 minutes longer. Strain into a clean pan and whisk in the remaining 4 tablespoons butter. Keep warm.
Arrange the steak skewers on the grill or under the broiler,* being careful that the bare ends of the skewers are not directly over (or under) the flame. Grill until medium rare, about 30 to 40 seconds each side.
Pour the sauce into a small bowl and set in the center of a large serving platter. Arrange the skewers around the bowl and serve immediately.
*Bring your broiler tray directly under the flame for best results.