In a small bowl, mix 100ml warm water, 20g sugar, and 2 tsp active dry yeast. Stir lightly and set aside for 5 minutes until it turns frothy and bubbly.

In a large bowl, combine the yeast mixture, 325g all-purpose flour, 30g sugar, 50ml warm water, ¼ tsp salt, and 40g room-temperature butter.

Knead for a few minutes until the dough turns soft and slightly elastic. Cover the bowl with plastic wrap and set in a warm place for about 1 hour, or until the dough doubles in size.

While the dough rises, heat 200ml milk and 2 tsp vanilla extract in a pan until just boiling.

In another saucepan, whisk 2 egg yolks, 30g sugar, and 15g cornstarch. Slowly pour in about one-fourth of the hot milk while whisking continuously to temper the yolks. Add the remaining milk and mix well.

Cook over low heat, stirring constantly, until the mixture thickens into a smooth custard. Remove from heat and add 50g chopped dark chocolate. Stir until melted, then add 50g cold butter and mix till glossy.

Transfer to a bowl, press cling film directly on the surface, and refrigerate until further use.

Once the dough has doubled, gently knead to remove air and divide it into 10 equal pieces. Shape each piece into a ball and place them on a lined tray with some space between.

Lightly flour the bottom of a glass or spoon and press into each ball to form a cavity. Cover loosely with cling film and rest for 20 minutes.

Fill each cavity with a spoonful of chilled chocolate custard. Brush the buns with egg wash or milk, sprinkle white chocolate chips, and bake in a preheated oven at 180°C (350°F) for 20–25 minutes, or until golden brown.

Transfer to a wire rack to cool completely. Serve slightly warm or at room temperature — the chocolate filling stays soft and luscious.