Activate the Yeast: In a bowl, mix lukewarm water with active dry yeast, sugar, and salt. Let it sit for 10 minutes until frothy.

Mix the Dough: To the all-purpose flour add softened butter and the yeast mixture. Knead until a soft dough forms. If the dough gets too dry add water as required to create a smooth and soft dough.

First Rise: Place the dough in a buttered bowl, brush the top with butter, and cover with plastic wrap. Let it rise for 1 hour or until it doubles in size.

Prepare the Custard Base: In a saucepan, whisk together milk, egg yolks, sugar, cornstarch/cornflour, vanilla extract, and a pinch of salt.

Cook the Custard: Heat the mixture over medium heat, stirring constantly, until it thickens.

Cool the Custard: Remove from heat, let it cool, and then transfer to a piping bag.

Release the Gas: After the dough has risen, punch it down to release the air.

Divide and Shape: Transfer to a floured surface, knead lightly, and divide the dough into 16 equal portions. Shape each into a ball.

Fill with Custard: Flatten each ball, fill with custard, and seal the edges tightly.

Create a Cavity: Press the center of each ball with a small flour-dusted cap to create a cavity.

Second Rise: Cover the buns with a damp towel and let them rise for another 15 minutes.

Preheat the Oven: Set the oven to 180°C (350°F).

Egg Wash: Brush the buns with beaten egg and, optionally, sprinkle sesame seeds on top.

Bake: Fill the cavities with more custard and bake the buns for 12-15 minutes until golden.

Final Touch: Remove the buns from the oven, brush them with butter, and transfer to a wire rack to cool slightly.