Begin by pouring the warm milk into the bowl of your stand mixer.

Add the sugar, cream, yeast, and sourdough starter, then stir gently until the sugar has fully dissolved and the mixture looks smooth and unified.

Add the two flours along with the salt. Mix at a low speed for about three minutes, then increase to medium speed and continue mixing for another three minutes to bring the dough together properly.

Lower the speed again and begin adding the butter a little at a time, allowing each piece to disappear into the dough before adding the next. Knead for another ten to twelve minutes, until the dough is silky, elastic, and no traces of butter remain.

Transfer the dough to a lightly oiled bowl, cover it, and let it rest in a warm place around 25°C until it has doubled in size. This usually takes about one to one and a quarter hours, though it may take longer if your kitchen is cooler.

Once risen, roll the dough out into a large rectangle, roughly 55 by 40 cm. Spread the chocolate custard evenly over the surface, then scatter the chocolate chips on top. Fold the dough over itself twice, like closing a letter. If the dough or filling feels too soft, place it in the freezer for five to ten minutes to firm up. Cut the dough into long strips, about nine in total.

Twist each strip several times, then coil it into a bun shape. Arrange the buns on trays lined with baking paper and leave them to rise in a warm spot for forty to sixty minutes, until soft and nicely puffed.

Heat the oven to 190 °C. Brush the buns gently with milk, or egg if you have it on hand. Bake for fourteen to sixteen minutes, until beautifully golden.

If you like a shiny finish, brush them while warm with a simple syrup made from two teaspoons of sugar dissolved in two teaspoons of boiling water.