1. Preheat oven to 180°C. Lightly brush just the bottom of an angle food cake mold with oil. Do not oil the sides.
2. Dissolve the matcha in the hot water in a heat-proof bowl or cup. Set aside and let cool to room temperature.
3. In a large bowl, whisk together the flour, baking powder, and salt. Using an electric mixer on high speed, beat the egg whites with 3 tablespoons sugar until the meringue holds medium-stiff peaks, making a meringue.
4. Spoon the meringue gently into a large bowl, taking care not to loose the air in it. In a separate bowl, beat the yolks and the remaining sugar on high speed until pale yellow and fluffy.
5. Reduce the speed to low and slowly add green tea and oil in a slow and steady stream on the side of the bowl. Beat until completely incorporated.
6. Working in three batches, gentle fold the dry ingredients into the egg yolk mixture.
7. Next, very gently fold in the egg white mixture in three batches as well.
8. Carefully pour the batter into mold and bake, turning about halfway through, at for about 50 minutes or when a toothpick comes out clean.
9. Let cake cool in the pan inverted/upside down on a cooling rack until completely cool.
10. Use a thin metal offset spatula or a thin knife to run along the sides and metal core of the pan to unmold.
11. Slice using a serrated knife.