Infuse the Milk: Scrape the vanilla bean seeds into the milk. Heat the milk gently in a saucepan just until warm (do not let it boil) to allow the vanilla to steep.

Whisk the Yolks: In a separate bowl, thoroughly mix the egg yolks and cane sugar until the mixture becomes pale and whitish.

Add the Flour: Sift the cake flour into the yolk mixture and whisk until completely smooth.

Temper the Eggs: Gradually pour the warm milk into the egg mixture while stirring continuously so the eggs don't scramble.

Thicken the Custard: Pour the entire mixture back into the saucepan over medium heat. Whisk constantly until the custard thickens into a rich, smooth cream. Remove the vanilla bean pods.

For the Chocolate Variation: If making chocolate custard, heat the milk plain. Whisk into the yolks, sugar, and flour mixture as above, and thicken on the stove. Once thick, remove from heat and immediately fold in the finely chopped chocolate until completely melted and glossy.

Cool Down: Strain the custards through a fine-mesh sieve into bowls for ultimate smoothness. Place plastic wrap directly touching the surface of the cream to prevent a skin from forming. Chill in an ice bath, then transfer to the refrigerator.

Combine Ingredients: In a large mixing bowl, combine the bread flour, cake flour, sugar, salt, and instant dry yeast. Pour in the egg and milk mixture.

Knead the Dough: Knead the dough until the surface is relatively smooth and no longer powdery.

Incorporate Butter: Add the room-temperature unsalted butter. Knead continuously until the dough absorbs the butter completely and becomes moist, smooth, and elastic.

First Proof: Form the dough into a ball, cover the bowl, and let it rest in a warm place until it has doubled in size.

Divide and Rest: Punch down the dough and divide it into 12 equal portions (roughly 74g–75g each). Roll them into neat, tight balls. Cover them and let rest for 15 minutes.

Second Proof: Gently reshape the balls to ensure they are perfectly round. Place them on individual, lightly floured squares of parchment paper. Cover and let them rise until doubled in size again.

Fry: Heat your cooking oil in a deep pot to 170°C (340°F). Carefully drop the donuts in (you can leave the parchment paper on the bottom to retain the shape, pulling it out with tongs once it detaches in the oil). Fry for 2 minutes on each side—4 minutes in total.

Coat in Sugar: Let the donuts drain for just a moment. While they are still warm (but safe to handle), toss them in granulated sugar to coat the outside.

Fill: Once the donuts have cooled completely, pierce the side with a chopstick or knife to create a cavity. Pipe in plenty of your chilled vanilla or chocolate custard cream.