Note: Meat that is partially frozen is easier to slice into thin strips. Asian chili sauce is available in most supermarkets under the name Sriracha. A chili-garlic sauce, known as sambal, could also be used; however, it is much spicier. Use 6-inch-long skewers for this recipe; you'll need about 24.
1. FOR SPICY PEANUT DIPPING SAUCE: Whisk the peanut butter and hot water together in a medium bowl. Stir in remaining ingredients. Transfer to a small serving bowl, and put aside to use after the beef has been placed on the skewers.
2. Cut the flank steak in half lengthwise and freeze it for 30 minutes.
3. Combine the soy sauce, oil, chili sauce, brown sugar, cilantro, garlic, and scallions in a measuring cup; set aside. Remove the flank steak from the freezer and slice each piece across the grain into 1/4-inch-thick strips. Weave the meat onto individual bamboo skewers. Dunk the meat end of each skewer in the marinade to coat; lay the skewers in a shallow dish, propping up the exposed ends of the skewers to keep them clean. Pour the remaining marinade over the meat. Refrigerate for exactly 1 hour.
4. Adjust an oven rack to the top position and heat the broiler. Lay the skewers on a wire rack set over a rimmed baking sheet and cover the skewer ends with foil. Broil for
6 to 7 minutes, flipping the skewers over halfway through, until the meat is browned. Serve immediately with the peanut sauce.