Combine plain flour, cake flour, and custard powder, in a mixing bowl, and mix well with a wooden spoon. Sift the flour mixture once, and set aside. In a separate mixing bowl, combine the brown sugar and white sugar, mix well, and set aside.
Using an electric mixer fitted with a whisk attachment, whisk eggs on high speed (speed 4 to 5 on my Kitchen Aid mixer) for 15 to 20 seconds. While whisking, gradually add in the combined sugars, in a steady, continuous stream. Continue to whisk until the egg mixture turns pale in colour, thickens, and triples in volume (ribbon stage - see Recipe Notes below), about 5 to 6 minutes.
Sift the flour mixture a second time. Divide the flour mixture into 3 portions. Fold the flour mixture, one portion at a time, into the egg mixture, using a light and quick hand. Cover the batter with a dry tea cloth, and set aside to rest for 1-2 hours.
Meanwhile, prepare the bamboo steamer. Line the base and sides of a 10-inch bamboo steamer with greaseproof paper. When batter has sufficiently rested, prepare enough water (for at least 30 minutes of steaming) in a deep pot and bring to boil, over high heat.
In a small bowl, combine evaporated milk, honey, baking powder, and baking soda. Fold into the batter until well incorporated. Lastly, add the oil, and gently fold into the batter, until well combined.
Pour the batter into the prepared bamboo steamer, cover with bamboo lid, set it stably over the pot of boiling water, and steam for 30 minutes, or until a bamboo skewer inserted into the centre of the cake, emerges free of sticky batter. Slice as desired, and serve immediately.