In a large mixing bowl, combine 1 cup of bubbly active sourdough starter, 3/4 cup of lukewarm milk, 1 large egg, 1/3 cup of granulated sugar, 1 tsp of salt, and 1 tbsp of instant yeast. Gradually mix in 3 3/4 cups of all-purpose flour and 10 tbsp of softened unsalted butter. Knead until smooth and elastic, about 6-8 minutes with a mixer or 10 minutes by hand.

Shape dough into a ball, place in a lightly greased bowl, cover, and let rise for about 3-4 hours or until visibly puffed up.

While the dough is rising, prepare butter block by pounding 8 tbsp of cold unsalted butter between parchment paper into a 7-inch square, chill again.

Roll the risen dough into a 10-inch square, place butter block in the center, fold corners over the butter, and roll out to a 10"x20" rectangle. Fold into thirds and refrigerate for 30 minutes.

Repeat the rolling and folding twice more, refrigerating for 30 minutes each time.

After final chilling, roll dough into a 12"x20" rectangle, sprinkle chocolate chips over half, fold other half over, shape into a loaf, and pinch seams.

Transfer loaf to greased 9x5 inch loaf pan, cover, and proof for approximately 2 hours or until puffed up.

Preheat oven to 375°F (190°C), bake loaf for 30-35 minutes until golden brown and internal temperature reaches 190°F (88°C).

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely for at least 1 hour before slicing.