Set a large skillet (preferably cast-iron) over medium heat. When it’s hot, put a nori sheet in the pan and toast until it turns slightly green, which will take only a few seconds. Turn and quickly toast on the other side. Set aside to cool and repeat with the other nori sheet.
While the pan is still hot, toast the sesame seeds, stirring or swirling the pan constantly to keep them from burning. When they are fragrant and beginning to turn golden, put them in a small bowl, sprinkle with the salt, and stir.
Crumble the nori into the bowl with the sesame seeds and salt. If you want a finer shake, whir the nori in a spice grinder for a few pulses and then add. Stir in the cayenne if you’re using it. Store in a tightly covered opaque container for up to a week.