In the bowl of a stand mixer, add warm milk, sourdough starter, eggs, and sugar. Mix on low speed with the dough hook attachment.
Add cocoa powder, espresso powder, salt, and 3 cups of all-purpose flour.
Let it come together into a soft dough.
Add the softened butter one tablespoon at a time.
Turn mixer to medium-low speed and knead until the dough is glossy and smooth and pulls from the sides of the bowl.
Cover with plastic wrap and let rise in a warm place for 8-10 hours.
Morning: Roll and Fill
Mix the butter, brown sugar, cocoa powder, and espresso powder together with a fork until it becomes a spreadable paste.
Turn the dough onto a lightly floured surface and gently press into a rough rectangle.
Use a floured rolling pin to roll into a 12×18-inch rectangular shape.
Spread the chocolate filling over the rolled-out dough.
Sprinkle chocolate chips over the top.
Roll up tightly from the long side.
To slice:
A sharp knife works great
But dental floss gives perfect swirl cuts without smashing the dough
Place rolls in a prepared baking dish lined with parchment paper.