Preheat the oven to 300F.
Mix milk and oil together and heat it until it is lukewarm.
Mash banana into the milk and oil mixture until smooth. You can also blend it.
Separate the egg whites from the egg yolks and add the egg yolks to the milk mixture. Then, sift and mix in the cake flour and black sesame powder.
In a separate bowl, beat the egg whites until foamy. Then add the sugar and continue to beat until stiff peaks form.
Fold a third of the egg whites into the egg yolk mixture. Once combined, continue with another third and then the remainder.
Transfer the batter into baking cups about 3/4 full, decorate with black sesame seeds, and bake for 25 minutes.
Remove the baking tray from the oven and drop the pan to help loosen the edges and preventing the cake from deflating in the center.
Let cool completely. Store in an air-tight container for up to 3 days at room temperature or for longer in the fridge.