Prepare the fillings. Cut the precooked octopus into 24 small pieces. Finely slice the scallions, and chop and drain the pickled ginger. Measure the sauce, mayonnaise, aonori, and bonito flakes so they are ready for serving. Use precooked octopus, not raw octopus, for these short-cooking takoyaki.
Whisk the batter. In a large bowl whisk together the flour, cornstarch, and baking powder. In a separate container whisk the eggs, cooled dashi, and soy sauce. Gradually pour the wet mixture into the dry ingredients, whisking until pourable and mostly smooth. The batter should be noticeably thinner than pancake batter; stir before every batch because the starch settles.
Preheat and oil the pan. Heat a takoyaki pan over medium heat according to its instructions. Brush each well and the surrounding flat surface generously with neutral oil. Set the pan on a stable, heat-safe surface. The oil helps prevent sticking and promotes a golden outside.
Fill the wells. Pour batter nearly to the top of each well. Add one piece of octopus, a pinch of scallions, a little pickled ginger, and, if using, a few tenkasu bits to each. Top up with a little batter if needed. Keep the add-ins small enough to fit inside a rounded ball.
Turn and cook the takoyaki. As the edges begin to firm, use takoyaki picks or wooden skewers to loosen the batter around each well. Rotate each piece about a quarter-turn, tucking the soft edges underneath. Continue turning and rounding the balls until the surfaces are golden and the egg-containing centers are cooked through, not liquid. Allow roughly 6–8 minutes per batch, adjusting for the size of the wells and the heat of the pan. Repeat with the remaining batter and filling. A 12-well pan usually needs two batches; timing varies. For egg dishes, a safe benchmark is 160°F (71°C) at the center if a thin food thermometer can be used.
Sauce and serve. Transfer the cooked takoyaki to plates, about six balls per person. Drizzle with takoyaki sauce and Japanese mayonnaise. Sprinkle with aonori and bonito flakes. Allow the centers to cool briefly before eating. Sauce just before eating; the centers retain heat and can burn the mouth.
