You can use 9 inch round bamboo steamer. If you use 8 inch, the cake will be taller and steaming time need to be longer too.
I highly recommend using a bamboo steamer lined with parchment paper to steam the cake. The bamboo basket has holes at the bottom and the cake cooks better that way and you won't have dense spot. Regular cake pan will work too, but if you have a bamboo basket, I would use that instead
Add the yeast (you can use active dry yeast or instant yeast) with 2 tbsp of the milk in a small bowl and let it bloom for about 10 minutes. It should be foamy and bubbly
Whisk the eggs, salt, and sugar vigorously until the sugar dissolves.
Add the milk, vanilla, and the yeast mixture and whisk again to combine.
Sift in the cake flour. Use whisk to combine. The batter may be lumpy. Strain the batter
Proof the batter:
Cover the bowl and let the batter proof at a warm place (about 82 F/28 C) and let the batter proof until foamy and bubbly, about 1 1/2-2 hours at the mentioned temperature. Try not to let it proof too long as the cake may taste sour. So, let the batter proof at a warm place so it won't take beyond 2 hours
Prepare the baking soda and baking powder mixture:
Fill your steamer with water and bring it to a boil. You want to fill it up with enough water but not too much that the water touches the bottom of the basket you are steaming the cake in, causing the bottom of the cake to be wet and has a dense layer
Use a whisk to gently stir the cake batter after proofing
Whisk oil, baking soda and baking powder in a bowl.
Scoop about 4 tablespoons of the cake batter into the oil mixture
Whisk vigorously to combine. We do this so that the consistency of the oil is similar to your fluffy cake batter so they mix much easier
Pour the oil mixture into the cake batter.
Use a spatula to swipe down and fold again to combine thoroughly
Pour the cake batter into a prepared bamboo steamer or cake pan.
Put some thin slices of almond or melon seeds on top if you like
Steaming:
If you don't use a bamboo steamer, make sure you wrap the lid of the steamer with a cloth to prevent water condensation from dropping on the cake surface. If you use a bamboo steamer, you won't have this problem
Steam on high heat for 45 minutes if using 8-inch round bamboo steamer and about 35-40 minutes if using 9-inch. If using metal cake pan, add another 5 minutes or so.
A cake tester inserted into the center of the cake should come out clean with a few crumbs is okay, but it shouldn't be wet and sticky. If it is, steam another 5 minutes and check again
Cool down:
Remove from the steamer. Let them cool down a little bit and then lift it out from the basket or pan onto a cooling rack so the bottom won't get soggy because of condensation trapped underneath. Serve the cake warm or at room temperature