Sprinkle 1 tablespoon of plain flour over the chicken mixture and stir well to coat. Pour in the 20 servings of chicken stock and bring the mixture to a boil, stirring constantly until it thickens slightly. Turn off the heat, then stir in the chopped 1 stalk spring onion and a 1 dash of pepper. Set this savory filling aside to cool completely.
In a separate mixing bowl, combine all the remaining ingredients: 2 eggs, 40 g ice water, 45 g flour, 150 g sponge cake mix, 150 g sugar, 1/2 teaspoon vanilla extract, and 2 teaspoons ovalette. Whisk at maximum speed until the batter becomes thick, pale, and fluffy, about 5-7 minutes. Lower the speed and gradually add the 60 g canola oil (or melted butter, if you prefer), mixing until just combined.
Prepare your cake pan by lining the bottom with non-stick baking paper and greasing the sides with a little butter. This will help the cake release cleanly after baking.