Set a bamboo steamer large enough to contain a 9x9 inch pan over simmering water. Sift together the cake flour, baking powder and green tea powder.
In another bowl, combine the egg yolks, sugar, water and vanilla extract. 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 pan.
Place cake pan in steamer. Stretch a kitchen towel over the cake pan without touching the surface, then cover with steamer lid. Steam cake for 20 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan 10 minutes, then turn onto wire rack to cool completely. Sprinkle with black sesame seeds for garnish if desired.