Preparation

1. Prepare your active sourdough starter by feeding it 4 to 6 hours before you start.
2. In a mixing bowl, stir together the milk, sourdough starter, egg, and sugar.
3. Add flour and salt, mixing until a shaggy dough forms.
4. Let the dough rest for 20 to 30 minutes and then knead lightly until smooth.
5. Incorporate softened butter gradually until it fully combines.
6. Cover and let sit at room temperature for a bit, then refrigerate overnight.

Lamination

1. Make a butter block by flattening cold butter between parchment paper.
2. Roll out the dough into a larger rectangle, place the butter block in the center, and fold the dough over it like a letter.
3. Perform three roll-and-fold rounds, chilling the dough 20 to 30 minutes between each round.

Shaping the loaf

1. Roll the final dough into a rectangle again and sprinkle the chocolate evenly over the surface.
2. Roll it up tightly, slice the log lengthwise, twist the two halves together, and tuck into a lined loaf pan.

Proofing and Baking

1. Let the loaf proof in a warm place for 3 to 5 hours, until puffy and slightly jiggly.
2. Preheat the oven to 375 F and brush the top with egg wash before baking.
3. Bake for 35 to 45 minutes, tenting with foil if browning too fast.
4. Allow to cool in the pan for 10 minutes, then transfer to a rack and don’t slice until cool.