In a large mixing bowl, combine warm whole milk, yeast, and sugar. Let it sit for about 5 minutes until foamy.

Gradually add all-purpose flour and vegetable oil. Mix on low speed for 5 minutes, then medium for another 5 minutes until smooth and elastic.

Transfer the dough onto a floured surface, roll into a log shape, and knead for about 6 minutes.

Let the dough rest for 10 minutes, then divide into 10 equal pieces and shape each into a smooth ball.

Whisk together eggs, whole milk, flour, and butter until smooth and thick yet pourable. Set aside.

Flatten each piece of dough into a disc, place a tablespoon of custard in the center, fold and seal the edges.

Arrange the sealed buns on a parchment-lined tray and let them rest for about 30 minutes.

Steam the buns for about 10 minutes, or until fluffy and glossy.