Sprinkle flank steak with salt and pepper. Combine coconut milk, sesame oil, chili sauce, garlic, ginger, sugar and cilantro in a bowl and whisk. Add flank steak to a ziplock bag or a shallow baking dish and pour marinade overtop. Refrigerate and let sit for 6-24 hours.
When ready to prepare, remove steak from fridge and let sit for 20-30 minutes. In a bowl, combine mango, shallot, jalapeno, salt, sugar and pepper, mix thoroughly, then set aside. Heat a large oven-safe skillet (like cast iron) over high heat. At the same time, preheat the broiler in your oven. (You can also grill the steak if you wish.) Using a pastry brush, brush oil onto the skillet so there is only a very thin layer. Remove steak from marinade with kitchen tongs, placing in the skillet. Sear on both sides for 2-3 minutes. Remove skillet and place is the oven directly under the broiler. Cook for 3-4 minutes, then flip and cook for 3-4 minutes more. This resulted in medium-medium well meat for us, but is totally dependent on the thickness of your steak and your desired doneness. Our steak was about an inch thick.
Remove skillet from over, and place steak on a cutting board to rest for 15-20 minutes. At this time, assemble your tortillas (warm them if desired), salsa, extra chili sauce for drizzling and cilantro. After the steak has rested, slice it into thin strips. Assemble tacos as desired and eat!