Set a bamboo steamer large enough to contain a 20cm cake tin over simmering water. Sift together the cake flour, baking powder and green tea powder. Set aside.
In a large bowl, combine the egg yolks, sugar, water and vanilla essence. Using an electric mixer, beat on medium speed until mixture has tripled in volume. Fold in the flour mixture, mixing just until combined.
Beat the egg whites until they form stiff peaks. Gently fold whites into the egg yolk mixture. Pour batter into the cake tin.
Place cake tin in steamer. Stretch a kitchen towel over the cake tin without touching the surface, then cover with steamer lid. Steam cake for 20 minutes, or until a toothpick inserted in the centre comes out clean. Cool on wire rack. Sprinkle with black sesame seeds and cut into pieces before serving.