Soak the split mung beans for at least 6 hours. Drain off the water and steam on high heat for about 30 minutes. If you can easily mash it with fork, then it's done. I put this in a blender and blend into a smooth paste
Place the mung bean paste, sugar, and oil in a large skillet. Cook until the sugar melted and the mung bean paste is smooth and doesn't stick to the skillet anymore. Remove from the heat and let it cool down completely , cover and store in refrigerator until ready to use. They can be stored in the fridge for up to 7 days
Preparing the dough:
Place the flour, instant yeast, sugar, and cooking oil in a mixing bowl. If you are using a stand mixer, use a dough hook attachment. Gradually add in milk or water. You may need more than 150 ml or not. Add more teaspoon by teaspoon until the dough comes together and continue to knead for 5 minutes. If you are kneading by hands, about 10-15 minutes until the dough is no longer sticky and smooth
Divide the dough into two equal portion. Add charcoal powder to the other dough and knead until the color is evenly distributed, which is black
Place both doughs on a lightly floured surface. Roll them into long logs and then divide equally into 16 pieces of yin and 16 pieces of yang. Reserve a tiny portion of yin and yang dough (for the dots)
Work with two pieces at a time and cover the rest to prevent drying. Place both doughs next to each other and then flatten the dough with your palm and use a small rolling pin to roll into a circle about 4 inches in diameter. So half will be white and the other half will be black.
Divide the mung bean paste into 16 equal balls or you can use a cookie scoop to scoop the filling on the center of the wrapper and gather all the sides to the middle and pinch to seal. Place it on a parchment paper seam side down. Continue with the rest. Pinch a very tiny dots from the dough you reserved earlier and place one on a black portion and the other on the white portion.
Place the steamed buns on a large tray cover with a large clean towel and let them proof until they rise to about 50% original size. Do not overproof the dough
Bring the water in a steamer to a rolling boil. Wrap the lid with a cloth. This will prevent water dripping from the lid and too much steam will make the bao has burn spots. Place some of the buns on the steamer and lower the heat to low and steam on LOW heat for about 8 minutes.
Turn off the heat and let the buns sit in the steamer for 1 minute and then remove from the steamer to let them cool down
I usually made this in a big batch and store the rest by placing them in a freezer bag. They can be stored up to 1 month. Place them on a baking sheet, not touching each other. Let them freeze for about 1 hour and then transfer to zip lock bag.
They can be reheated on the steamer over medium-high for about 5 minutes and as good as new
You can increase the amount of sugar and oil in the mung bean paste filling. I half the amount of sugar in the recipe as I don't want it to be too sweet and if you add more oil, it should be even smoother. It's your call!
The sweet mung bean paste can be used for Chinese moon cake, ang ku kueh, or other Asian pastries and desserts
You can also replace all 300 gr of all-purpose flour with cake flour or premixed Hongkong/Vietnamese bao flour and omit the wheat starch/corn starch.
Yin Yang Sweet Mung Bean Paste Steamed Buns https://whattocooktoday.com/yin-yang-sweet-mung-bean-paste-steamed-buns.html