In a large bowl, whisk together 200g plain flour, 800ml dashi stock, 1 large egg, 1 tsp soy sauce and a pinch of salt until smooth and free of lumps.
Leave the batter to rest for 15–20 minutes to allow the flour to fully absorb the liquid.
Cut 200g cooked octopus into 1–1.5cm pieces and finely slice the green parts of 4–5 spring onions. Have 50g tempura scraps (tenkasu) to hand.
Heat the takoyaki pan over medium-high heat and generously grease each mould with vegetable oil.
Pour the batter into each mould until just below the rim, then add a piece of octopus, some sliced spring onions and a few tempura scraps to each.
When the edges begin to set and turn golden (after 2–3 minutes), use cocktail sticks to turn each one 90° and form a ball.
Continue cooking for another 5–7 minutes, turning them occasionally, until they are crisp and golden on the outside.
Remove the takoyaki from the pan and transfer to a serving plate. Drizzle with takoyaki sauce and Japanese mayonnaise, then sprinkle over dried bonito flakes (katsuobushi) and aonori. Serve immediately.