Finely chop or mince the cabbage.
In a bowl, mix chopped cabbage, eggs, dashi powder, plain flour, baking powder and water. Add a pinch of salt and grated ginger.
Mix well and you can start baking. If the batter seems too thick, add more water.
Heat oil in a pan. Spread the batter evenly on the pan. I always keep a few spoons of the batter in the bowl and first I will put prosciutto (or ham) on top and then cover the prosciutto here and there with the rest of the batter so that it doesn't burn when the okonomiyaki is baked from the other side.
Cover the pan with a lid and let it cook on a low heat. When the bottom is a little toasted, I add a little water. It depends on how thick/thin the okonomiyaki is. When the water evaporates, turn the pancake to the other side and let it bake.
To serve: Spread the okonomi sauce on top, drizzle with Japanese mayonnaise, and sprinkle with dried bonito flakes and aonori.