In the bowl of a stand mixer, mix together the sourdough starter, warm milk, eggs, sugar, bread flour, and salt. Use a dough hook to mix for 5–6 minutes until the dough is smooth and elastic.

Add the softened butter and knead until fully incorporated. Cover the bowl and let it rest at room temperature for 4–6 hours, then refrigerate overnight.

Roll out the chilled dough into a large rectangle. Place the cold butter block in the center of the dough.

Fold the dough over the butter like a letter. Turn the dough and repeat this fold three times, chilling for 30 minutes between each fold.

Roll the laminated dough into a 10×20 inch rectangle. Evenly sprinkle mini chocolate chips over the dough.

Roll it tightly into a log, then slice it in half lengthwise. Twist each half into a spiral and place them in greased muffin tins.

Let the cruffins rise at room temperature for 4–6 hours, or until they are doubled in size and puffy.

Preheat the oven to 375°F (190°C). Bake the cruffins for 25–30 minutes, or until they are golden brown and crisp.

Let the cruffins cool slightly before topping or filling them with vanilla pastry cream or custard if desired.