Pre-heat oven to 165°C (330°F) with baking shelf placed on the lowest rack in the oven. Do NOT grease the cake pan.
In a medium mixing bowl, combine optima flour, Hong Kong flour and baking soda and stir with a whisk to mix well.
In another mixing bowl, place egg yolks, water, vegetable oil, butter oil (or vanilla extract). Use a whisk to stir until well mixed. Sprinkle in flour-soda mixture from step (2), 1 to 2 tbsps at a time, stirring with a whisk to mix well, before adding more. Mix until mixture is smooth and free of lumps.
Place egg whites and cream of tartar into a dry and grease-free mixer bowl. Using a hand-held or electric mixer fitted with a whisk attachment, whip on high speed until egg whites turn frothy. Sprinkle in the sugar a little at a time, and continue to whip until stiff peaks form.
Add one-third of the whipped egg white mixture to the wet flour mixture from step (3), fold gently by hand with a spatula. Then add all the remaining whipped egg whites and fold gently, moving always in one direction, until well mixed.
Pour out into prepared cake pan and level the surface. Gently tap the pan on the counter a few times to eliminate air pockets. Bake at 165°C (330°F) for 50 to 60 minutes, or until done.
To test: (a) Insert a skewer into the centre of the the cake, it is done when it emerges clean and free of sticky batter; AND/OR (b) press your fingers gently on the centre, the cake is done if it feels pillowy and springs back to the touch; AND/OR (c) insert an instant-read thermometer into the centre of the cake, the cake is done when it's internal temperature reaches 98°C (210°F).
Once immediately out of the oven, invert the cake pan onto a cooling rack, and let it 'hang' for 15 minutes. Turn the cake pan back on its base, run a knife around the pan to loosen the sides and remove the cake. Allow to cool to room temperature.
Meanwhile, if you choose to wash your strawberries, make sure that they are patted very dry with paper towels. Choose 13 (more or less) of the nicest strawberries for decorating the top of the cake. Slice the rest for the filling.