Prepare the steamer by bringing the water to a boil. Line muffin cups with paper liners. Do not use silicone cups as they do not conduct heat well and the cake won't crack and smile.
Whisk all-purpose flour, baking powder, and salt (omit if using self-raising flour) to combine. Combine milk and sugar and stir until sugar dissolves. Add the oil and whisk to combine. Sift in all-purpose flour, baking powder, and salt (omit if using self-raising flour). Whisk until just combined. Do not overmix. Just combine until you no longer see pockets of flour
Remove about 120 grams of the batter into a separate bowl. Add a few drops of green food coloring and a few drops of red food coloring to the main batter. Add black sesame seeds to the red batter. I transfer the red batter into a measuring cup with a spout for easy pouring
Divide the red batter evenly into the muffin cups, they should be about 85% full at this point. Spoon some of the green batter evenly on top of the red. It should be about 90% full at this point. Dip a toothpick into a green food coloring and draw some lines to resemble the patterns on watermelon skin
Steaming:
Put the muffin cups inside the steamer and make sure to wrap the lid with a cloth to prevent water condensation from dripping onto the cake
Steam over high heat for 20 minutes. Do not open the lid during the steaming process
Cool down:
Let the huat kueh cool in the cups for about 5 minutes and then remove from the cup onto a cooling rack and let them cool down completely
Storage:
The huat kueh can be stored in an air-tight container for 3 days. For longer storage, wrap it individually with a cling wrap and put in a freezer bag. They can be kept frozen for one month
Simply reheat in the steamer or microwave until they are warm