Whisk the batter. In a medium bowl, combine the flour, baking powder, and sugar. Slowly pour in the dashi water and whisk until the batter is smooth. Let it rest for 10 to 15 minutes if time allows; this helps the flour hydrate for a better texture.
Prep the mix. In a large mixing bowl, combine the shredded cabbage, thinly sliced chicken, eggs, and pickled ginger. Pour the batter over the mixture and fold everything together until the cabbage and chicken are evenly coated.
Cook the pancake. Heat a lightly oiled non-stick skillet over medium heat. Pour the mixture into the center and shape it into a circle about 2 cm thick.
Sear and steam. Cover the pan with a lid and cook for about 5 minutes. Using a lid is important here because it creates the steam needed to cook the chicken and soften the cabbage while the bottom develops a golden-brown crust.
Flip. Carefully flip the pancake over. Cook, covered, for another 5 minutes until the second side is browned, the chicken is fully cooked through, and the pancake feels firm.
Prep the sauce. While the pancake cooks, whisk the oyster sauce, ketchup, Worcestershire, and honey together in a small bowl.
Garnish and serve. Brush the top of the okonomiyaki generously with the sauce. Drizzle with mayonnaise in a zigzag pattern and top with a handful of fresh green onions.