To make cake, preheat oven to 200°C. Grease 10" x 7" cake pan with butter. Line with 14" x 7" parchment paper, making sure paper has no air pockets or creases. (Use 13" x 10" pan if recipe is doubled.)

Measure and prep ingredients as detailed above.

Add water to green tea powder. Stir till smooth. Cover. Set aside.

Whisk egg yolks with 10 g castor sugar till thick and pale. Add corn oil. Whisk till thick. Add green tea paste. Mix till even. Sift cake flour into mixture in 2 batches. Mix till just even after each batch.

Whisk egg whites on medium-low speed till very frothy. Add cream of tartar. Whisk till thick foam forms. Gradually add 20 g castor sugar whilst continuing to whisk. Reduce speed to low. Keep whisking till firm, hooked peak stage.

Loosen egg yolk mixture from bottom of bowl. Add egg whites in 2 batches. Mix with whisk till almost even after each batch. Scrape down and fold with spatula till just evenly mixed, banging bowl against worktop 2-3 times.

Bake cake on middle shelf of oven till risen, about 9 minutes (10 minutes if recipe is doubled). Place baking tray in bottom of oven. Continue baking till crust is lightly brown and middle of cake doesn't make squishing sound when pressed, 4-5 minutes (5-6 minutes if recipe is doubled), checking once every 30-45 seconds towards the end.

Remove pan from oven. Drop from 1' high 2-3 times. Loosen cake with knife. Drag onto wire rack. Leave till cold. Remove from wire rack. Place new parchment paper on cake. Flip cake upside down. Peel old parchment paper from cake, slowly. Flip again.

To make filling, whip cream till thick enough to hold its shape. Add green tea powder. Whisk till thick enough to stick to whisk.

To assemble Swiss roll, spread cake with whipped cream, then red beans. Roll. Refrigerate for 2 days, covered. Remove from fridge. Cut with serrated knife, wiping knife clean after each cut. Tuck in when cake is soft but filling is still firm.