In the bowl of your stand mixer, add warm milk, sugar, cream, yeast and sourdough starter. Stir until the sugar dissolves and everything is evenly mixed.
Add both flours and the salt. Mix on low for 3 minutes and medium speed for 3 minutes.
Go back to low speed and add the butter little by little.
Continue kneading for 10-12 minutes more until smooth and the butter fully incorporated.
Place the dough in a lightly oiled bowl, cover, and let rise in a warm spot (25–28 °C) for 1 ¼ - 1 ½ hours until doubled.
Roll the dough into a 55 × 40 cm rectangle.
Spread the chocolate custard evenly and then the chocolate chips.
Fold the dough twice like an envelope. Place in the freezer for 5-10 minutes if the dough/custard feel too soft.
Slice into long strips (approximately 9 strips).
Twist each strip several times and shape into a roll. Place on baking paper lined trays.
Let rise in a warm place for 40–60 minutes, until puffy.
Preheat your oven to 190°C.
Gently brush the buns with milk.
Bake for 14-16 minutes until golden brown.
Optional: brush with syrup (2 tsp sugar mixed with 2 tsp boiling water) for a glossy finish.
