Make the Custard Cream: In a saucepan, combine the milk and pastry cream powder. Mix well and turn on the heat. Stir constantly until the mixture thickens, and then cook for a couple of extra minutes. Once done, pour the cream into a pastry bag and allow it to cool.

Prepare the Dough: In a bowl, mix the boiled and mashed potato, egg, 1/2 tablespoon of milk, oil, yeast, sugar, and salt until smooth. Gradually add flour, using your hands to knead the mixture. Aim for a dough that is soft and pliable (this should take about 15 minutes).

Let the Dough Rise: Cover the bowl with a tea towel and place it in a warm area to rise until doubled in size.

Shape the Buns: Once risen, sprinkle flour on your surface, shape the dough into buns, and place them on a baking sheet. Let them rise again until puffed.

Prepare for Baking: Make a cut on the top of each bun using a sharp blade. Brush each with egg yolk mixed with a drop of milk.

Fill with Cream and Bake: Fill each bun with the custard cream and bake in a preheated oven until golden brown.

Optional: For an extra treat, brush the warm buns with condensed milk and sprinkle with grated toppings.