In a large bowl, combine the soy sauce, brown sugar, sesame seeds, garlic and ginger. Add the beef and stir to coat. Cover and marinate for 30 minutes.
In a bowl, stir together the vinegar, granulated sugar, salt and gochugaru. Add the cucumber and stir to coat. Cover and store in the refrigerator until ready to serve.
Remove the meat from the marinade and transfer the marinade to a saucepan. Bring to a simmer over medium-low heat and cook until reduced by half.
Heat the oil in a large skillet set over medium heat. Add the beef and cook, stirring, until just cooked through, 2 to 3 minutes. Transfer the beef to the reduced marinade and stir to coat.
Shape the rice into a skull shape and place on a large platter. Use the cucumber slices to create eyes, a nose and mouth on the skull. Place the beef at the base of the skull to serve.
Gochugaru is a Korean chile powder that can be found in specialty Asian markets.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.