1. In a bowl, combine the water, milk, yeast, sugar, and oil and mix for 1 minute. Cover with a towel and let it sit for 10 minutes until the yeast starts to activate.
2. Meanwhile, in the mixer bowl, combine the flour, baking powder, and salt. Once 10 minutes have passed, add the yeast mixture to the mixer bowl. Using the dough hook, start the mixer on low speed for 1 minute and then increase to medium speed for 2 minutes until the dough forms. On a baking tray, place the duck thighs and brush them with hoisin sauce and the remaining ingredients. Cover with parchment paper and aluminum foil and bake in a preheated oven at 140°C for 1 hour. After the hour, remove the paper and foil and continue baking for 30 minutes. Once the bone easily separates from the meat, shred the cooked duck with a fork. If you want the filling to be moister, add another 1-2 tablespoons of hoisin.
3. Continue kneading for 5 minutes until the dough is soft and elastic. Remove the dough from the mixer and form it into a ball. Place it back in the bowl, cover with plastic wrap, and let it rest in a warm place for about 2 hours until the dough has risen well.
4. Once the dough is ready, lightly flour your work surface and roll it out with a rolling pin to a horizontal size of about ½ centimeter thick. Using a round cutter of about 8 centimeters, cut the dough to get as many bao buns as possible. If you don't have a cutter, use a glass. Brush the surface of the baos with sunflower oil and fold them into a crescent shape. Press each bun with the rolling pin and place it on parchment paper. Cover with a towel and let them rise for about 30 minutes.
5. While waiting for the baos to rise, put water in a pot and place the bamboo steamer basket on top, lining it with greased parchment paper. Bring the water to a boil, making sure the bamboo does not touch the water. Place 5 baos at a time in the bamboo, cover with the lid, and steam for 12-15 minutes. When they are ready, remove them from the bamboo and fill them with the duck filling.
