Sieve plain flour, baking powder, baking soda, and custard powder into a large bowl. Mix well.
In a separate bowl, whisk eggs with caster sugar and dark brown sugar until light and fluffy.
Gradually add milk and vegetable oil to the egg mixture, mixing until just combined.
Pour the wet ingredients into the dry ingredients and gently fold until just combined. Do not overmix.
Pour the batter into a greased and floured 8-inch round cake pan or a heatproof ceramic bowl.
Steam the cake over high heat for 35-40 minutes, or until a skewer inserted into the center comes out clean.
Remove from steamer, let it cool slightly before slicing and serving warm.
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