I used the sourdough discard that I had accumulated for weeks and keep them frozen. I simply thaw them overnight in the fridge before I planned to use them the next day
Prepare the cake batter:
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
Mix the starter with oil and set aside. Crack egg into a mixing bowl of a mixer. Add sugar and use a whisk attachment to cream until the mixture is pale and creamy, about 2 minutes
Add the starter gradually alternating with the flour mixture. Mix until just combined. Don't overbeat. It's okay to have some flour on the side of the bowl. We can fold that in manually with a spatula. Lastly add the raisins. The batter is thick at this point
Scoop the thick batter to the cup with a spoon. You might need a rubber spatula to help you. Fill it up to about 3/4 full
Steaming:
Wrap the lid of your steamer with a cloth if you don't use a bamboo steamer. This is to prevent water condensation from dripping on the surface of the cake
Place the cups inside the steamer and steam over high heat. Make sure the heat is 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
Notes
Because my sourdough starter is made with bread flour, which has 12.7% protein content, I decided to use cake flour for the rest of the flour calls for in the recipe to make it softer and fluffier. Depending on what kind of flour you use to make your starter, you can adjust accordingly.