Make filling: Steam the egg yolks for 10 minutes, then mash them into fine crumbs. In a medium bowl, beat the butter until softened. Add the yolks and the remaining ingredients. Mix until smooth. Cover and refrigerate to firm for about 45 minutes.

Divide the filling into 14 equal portions (about 13g each). Roll each into a smooth round ball. Keep them in the fridge until needed.

Make dough: Combine warm water (about 110°F/43°C) and 4 grams of sugar. Sprinkle the yeast over. Let sit for 5-10 minutes until it gets foamy and activated.

In a large bowl, whisk the flour, salt, and 8 grams of sugar until combined. Make a well in the center. Pour in the yeast mixture and oil, then mix until the dough comes together.

Transfer onto a work surface. Knead the dough until it becomes elastic and smooth for up to 20 minutes.

Divide the dough into 14 equal portions (about 33g each). Roughly shape each dough into a ball. Cover and rest them for 15 minutes to relax the gluten.

Take 1 dough. Roll out the edges keeping the center thicker. Place a portion of the chilled filling onto the center and wrap it. Tightly pinch the seam to prevent any leakage.

Round it into a ball and place onto a piece of parchment paper. Set aside with a cover. Repeat with the rest.

Wait 10 minutes after the heat is off before opening the lid. This will prevent the buns from collapsing. Serve warm to get that tasty molten filling.