Preheat the oven to 350 degrees F. To make the togarashi, toast the nori on an ungreased baking sheet for 20 minutes in the oven. Allow to cool, then rip the nori into pieces that will fit in your spice mill. Grind the nori, sesame seeds, orange zest, and pepper flakes in batches if necessary, and mix together in a small mixing bowl. Set aside.
Heat the oil in a wok over high heat until very hot. Add the prawns and cook 3 to 4 minutes, tossing and turning the prawns as they cook. When they are firm and pink, sprinkle the prawns with the togarashi, salt, and pepper, and toss well to coat.
You can also brush the prawns with the oil, sprinkle them with the togarashi, salt, and pepper, and grill or broil them, turning often, until firm and pink, 3 to 4 minutes.
Recipe courtesy of Tom Douglas' Seattle Kitchen (William Morrow, 2001)