Mix all dry ingredients in a mixing bowl. Set aside. Bring a water in the steamer to a boil. Make sure there's enough water in the steamer. Line individual muffin cups with paper
Crack an egg into a mixing bowl of a mixer. Add sugar and use a whisk attachment to cream until the mixture is pale and creamy with medium speed, about 2 minutes. Add milk and mango puree. Continue to whisk another 2 minutes
Gradually add in the flour mixture, alternating with the cooking oil. Continue to do so until you run out of flour and oil. The batter will be thick
I transferred the batter into a piping bag as I feel like it's easier to fill up the cup without being messy, but you can use a spoon to scoop the thick batter into the cup with a spoon. Fill it up to about 3/4 full. Smooth the top of the batter to make it neat so it will "split" open neatly
If the lid you use to cover the steamer's pot doesn't have a dome shape where there water condensation can drip down on the side, I suggest you wrap the lid with a cloth. If you use a bamboo steamer or if the lid is dome, you don't need to wrap with a cloth
Place the cups inside the steamer and steam over high heat. Make sure the heat is high. I use the largest burner on the stove and the knob is set to high. Steam for 15 minutes. Do not open the lid throughout the steaming process. After 15 minutes, you can open the lid immediately and remove the cake from the steamer. Let them cool down for 1 minute and then transfer out from the muffin tin and let them cool down on a cooling rack