Steam the duck egg yolks over medium heat for 15 minutes. Mash into a paste.

In a small saucepan, add the mashed duck egg yolks, 2/3 of the milk, gelatin powder, fresh egg yolk, melted butter, sugar, and custard powder. Mix well. Cook over low heat, stirring constantly until the mixture thickens into a paste.

Remove from the heat and stir in the reserved milk until combined. Chill in the fridge for 1 hour. Form into 8 equal-sized balls, cover, and chill in the fridge.

In a mixing bowl, combine the flour, sugar, yeast, and baking powder.

Add warm water and vegetable oil to the dry ingredients. Knead the dough until it becomes smooth and elastic. Cover the dough and let it rest for 30 min.

Divide the dough into 8 equal parts. Flatten each dough ball into a circle, leaving the center thicker than the edges.

Add a ball of egg yolk filling to the center of each dough circle. Press it down to compact tightly. Gather the edges of the dough and pinch them up and together, twisting the dough to seal the filling inside.

Place each bun on a piece of parchment paper with the uneven side down and shape them taller with your hands. Cover and let them rest for 10 minutes.

Steam the buns over high heat for 8-10 minutes, until they become fluffy and fully cooked. Then turn off the heat and allow them to rest for 10 minutes.

Serve the buns warm and enjoy the delicious, oozing egg custard filling!