Measure and prep ingredients as detailed above. Bring to room temperature. Preheat oven to 160°C.
Blend 175 g bananas, egg yolks, corn oil and 15 g castor sugar till smooth and thoroughly incorporated. Sift cake flour, baking soda and baking powder into mixture. Add salt. Mix with whisk till just even.
175 g very ripe bananas (weigh without peel), 60 g egg yolks, 40 g corn oil, 15 g castor sugar, 70 g cake flour, 1/2 tsp baking soda, 1/4 tsp baking powder, 1/8 tsp salt
Separately whisk egg whites till frothy. Add cream of tartar. Whisk till egg whites form thick foam. Gradually add 50 g castor sugar, still whisking. Continue to whisk till egg whites reach firm peak stage.
140 g egg whites, 1/16 tsp cream of tartar, 50 g castor sugar
Add egg whites to yolk mixture in 3 batches. Mix with whisk till almost even after each batch. Scrape down and fold with spatula till just evenly mixed, banging mixing bowl against worktop 2-3 times.
Pour batter into 18 x 8 cm 2-piece chiffon pan that's not non-stick, rotating pan as you pour so that batter is spread evenly. Jiggle pan till batter is level.
Bake cake in bottom of oven for about 1 hour, till well browned and, when pressed lightly, springs back and squishes softly. (Loud squishing means the cake is still too wet.)
Remove cake from oven. Invert onto something narrow and tall, e.g. jam jar or inverted glass. Leave till cool.
Cut cake out of pan. Serve.