Stir together in a small bowl soy sauce, ginger, brown sugar, sesame oil, cayenne and garlic.
Pour marinade into a large ziploc bag, then add steak and turn to coat. Refrigerate for 1 to 4 hours turning bag several times.
Heat a ridged grill pan over medium-high heat until very hot. Remove the steak from the bag, pour the marinade into a 1-quart saucepan and reserve. Pat the steak dry with paper towel and cook for 6 to 8 minutes per side for a medium rare steak.
Remove steak to a cutting board and let stand for 10 minutes.
Add 2 tbsp of water to the marinade in the saucepan and heat to boiling over high heat, boil for 2 minutes.
Slice steak into thin strips diagonally against the grain. Serve with the cooked marinade.