To make cake, trim 5 mm thick corrugated cardboard to fit sides of 23 x 15 cm cake pan. Wrap each piece of cardboard in aluminium foil, shining side facing out. Line bottom of 20 x 12 x 7.5 cm cake pan with 2 layers of parchment paper.
Preheat oven to 160°C. Measure ingredients for cake and prepare green tea as detailed above.
Add 20 g boiling water to green tea powder. Mix till smooth. Add 30 g room temperature water, mix thoroughly, set aside to cool down
20 g boiling water, 5 g green tea powder, 30 g room temperature water
Whisk 10 g castor sugar with egg yolks till dissolved. Add corn oil. Whisk till just combined. Add green tea. Whisk will just thoroughly mixed. Sift cake flour into mixture. Whisk till just thoroughly mixed but small lumps are still visible.
10 g castor sugar, 60 g egg yolks, 30 g corn oil, 40 g cake flour
Separately whisk egg whites till frothy. Add cream of tartar. Whisk till thick foam forms. Gradually add 45 g castor sugar whilst still whisking. Continue to whisk till firm peak stage.
105 g egg whites, 1/16 tsp cream of tartar, 45 g castor sugar
Thoroughly whisk yolk mixture, which should be smooth now. Add half of egg whites. Mix with whisk till almost even. Add remaining egg whites. Mix with whisk till almost even. Scrape down and fold with spatula till just evenly mixed, banging mixing bowl against worktop 2-3 times.
Pour batter into 20 x 12 x 7.5 cm cake pan, slowly and from about 30 cm high. Jiggle pan till batter is level.
Place cake pan holding batter in 23 x 15 cm cake pan. Tuck cardboard between 2 pans. Bake till middle of cake doesn't make squishing sound when pressed gently, about 55 minutes.
Remove pans from oven. Remove cardboard and outer pan. Drop pan holding cake from about 30 cm high, 2-3 times. Invert pan onto wire rack. Leave till just cool.
As soon as cake is cool, slide knife along sides of pan. Unmould cake. Discard both layers of parchment paper.
To make whipped cream, whisk cream till just thick enough to hold its shape. Add green tea powder. Whisk till evenly mixed and cream is dead stiff.
200 g dairy cream, 1 tsp green tea powder
To assemble, cut cake horizontally with serrated knife or cake slicer into 2 equal halves. Set aside bottom half of cake. Invert top half so that cut side faces up. Spread with half of whipped cream. Top with remaining half of cake, cut side down. Spread with remaining whipped cream. Smooth top. Trim about 1 cm from 4 edges.
Transfer cake to serving plate. Refrigerate till ready to serve. Enjoy trimmings whilst waiting.