Whisk the soy sauce, honey, and wasabi powder together in a glass container. Transfer 1/3 cup of the liquid to a small bowl and reserve for dipping. Place tuna in the remaining marinade, turn to coat, and refrigerate for at least 1 hour and up to 4 hours, turning once halfway through.
Fill a chimney starter with natural chunk charcoal and light.
Remove the tuna from the marinade and pat dry. Sprinkle the sesame seeds onto a dinner plate and roll the tuna pieces in the seeds, pressing down lightly to evenly coat.
Place a grill grate on top of the chimney starter to heat for at least 1 minute. The charcoal should have burned down to one fourth of its original size and be at least 8 inches from the grill on top.
Once it's hot, carefully move the grate off the heat and brush with the oil. Return to the chimney, place the tuna on the grill, and cook for 30 seconds per side for a total of 2 minutes cooking for rare. Add an additional 15 seconds per side for medium. Remove the pieces to a clean dinner plate, cover with plastic wrap, and rest for 3 minutes. Thinly slice the tuna and serve warm with the dipping sauce, or chill and serve cold.