In a small bowl, combine warm milk and instant yeast. Set aside.
In a medium mixing bowl, add all-purpose flour, sugar, and salt. Mix until combined.
Add vegetable oil.
Slowly add the yeast mixture to the dry ingredients. Use a pair of chopsticks to mix until combined. Then, use your hands to knead it into a dough.
Separate the dough into 8 equal portions. Shape 7 of them into small round balls. Take the remaining dough and divide it into 7 equal portions, shaping each into a raindrop.
In a small bowl, combine cacao powder and hot water. Prepare a pan of warm water and place your bamboo steamers on it. Arrange the dough balls in the steamer.
Brush a single layer of chocolate sauce onto each round dough ball. Cover with the lid and let them rise for about an hour or until they are 1.5 times their original size. The dough is done rising once the mushroom "skin" is formed and cracked.
Steam the buns for 20 minutes on medium-high heat. Turn off the heat and let them sit for 5 minutes without opening the lid.
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