In a large mixing bowl, combine your active sourdough starter, lukewarm whole milk, large egg, granulated sugar, salt, and instant yeast. Stir until well incorporated.

Gradually add the all-purpose flour and softened unsalted butter. Knead for 6 to 8 minutes until smooth and elastic.

Cover the dough and let it rise at room temperature for 3 to 4 hours until doubled in size.

Prepare the cold butter by placing it between two sheets of parchment paper and pound it into a 7x7 inch square. Refrigerate until firm.

Roll the risen dough into a 10x10 inch square and position the chilled butter block in the center. Fold corners over the butter.

Roll the dough into a long rectangle measuring approximately 10x20 inches. Fold into thirds like a letter and refrigerate for 30 minutes.

Remove dough and repeat the rolling and folding process two more times, chilling for 30 minutes after each fold.

After the final chill, roll the dough into a 12x20 inch rectangle and sprinkle semisweet chocolate chips evenly across the surface before folding the dough to enclose the chips.

Place the shaped loaf into a greased loaf pan. Cover and proof at room temperature for about 2 hours until puffy.

Preheat oven to 375°F (190°C) and bake the bread for 30 to 35 minutes until golden brown.

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