Follow these simple steps to create your very own Steam Custard Cake:

1. Prepare the Steamer: Fill a large pot with water and bring it to a boil. Place a steaming rack inside.
2. Mix Dry Ingredients: In a mixing bowl, combine flour, sugar, cocoa powder (if using), baking powder, and salt. Whisk until well combined.
3. Combine Wet Ingredients: In another bowl, whisk together the milk, vegetable oil, eggs, and vanilla extract.
4. Mix Together: Gradually add the wet ingredients to the dry ingredients, stirring until everything is well combined and smooth.
5. Incorporate Custard: Gently fold in the custard until evenly distributed within the batter.
6. Prepare the Pan: Grease a steam-safe cake pan with a little oil.
7. Pour the Batter: Pour the batter into the prepared pan, smoothing the top.
8. Steam the Cake: Place the pan on the steaming rack. Cover it with a lid and steam for 30-40 minutes, or until a toothpick inserted in the center comes out clean.
9. Cool the Cake: Carefully remove the cake from the steamer and let it cool in the pan for about 10 minutes.
10. Unmold and Serve: Gently run a knife around the edge to loosen, then turn it upside down onto a plate. Your Steam Custard Cake is ready to be enjoyed!