Mix sourdough starter, flour, water, and sugar together in a jar until fully combined.

Cover loosely and rest at room temperature for 12 hours until doubled in size.

In a bowl, cream together softened butter, brown sugar, and powdered sugar until combined.

Mix in sourdough starter and vanilla extract until well incorporated.

In a separate bowl, whisk together flour, cornstarch, salt, and baking soda.

Gradually add dry ingredients to wet ingredients, mixing until just combined.

Fold in mini chocolate chips.

The mixture should be thick but spreadable, similar to very thick frosting.

If too thick, add milk one teaspoon at a time. If too thin, add flour one tablespoon at a time.

Wrap tightly in plastic wrap and refrigerate overnight for at least 12 hours.

Whisk freshly brewed hot coffee and cocoa powder together until smooth.

Cover and let bloom for 1 hour before using.

In a large mixing bowl, combine sweet levain, bloomed chocolate, water, sugar, and bread flour.

Mix until just combined and let rest for 30 minutes.

Add salt and vanilla extract to the rested dough.

Mix thoroughly until fully incorporated.

Let rest for another 30 minutes.

Fold chocolate chips evenly throughout the dough.

Let rest for 30 minutes.

Perform stretch and folds every 30 minutes for 2 hours total (4 sets).

After the final set of folds, let the dough rest at room temperature for 6 to 8 hours until it has increased in size by about 50 percent.

After bulk fermentation is complete, gently stretch the dough out on a lightly floured surface into a rectangle approximately 10 by 16 inches.

Divide the cookie swirl filling in half.

Place the first half between two pieces of parchment paper and roll it out into a rough rectangle about 10 inches long and 5 to 6 inches wide.

Lay the first cookie swirl layer in the center of the dough rectangle.

Fold each side of the dough up into the middle, fully covering the first cookie swirl layer.

Roll the second half of cookie swirl between the same parchment paper into a rectangle about 10 inches long and 5 inches wide.

Lay the second cookie swirl layer on top of the folded dough.

Gently roll the entire dough from one short edge to the other, creating a tight spiral with cookie swirl running throughout.

Place the shaped loaf seam side up in a large proofing basket or floured bowl lined with cloth.

Cover and refrigerate overnight for 12 to 16 hours.

Preheat oven to 450 degrees F (230 degrees C).

Remove the proofing container from the refrigerator.

Place a piece of parchment paper over the top of the proofing container.

With one hand on the parchment and one hand supporting the bottom, flip the loaf onto the parchment.

Place the loaf inside the baking pan.

Cover with the second loaf pan as a lid.

Bake covered for 9 minutes.

Remove the lid and score the bread. Replace the lid.

Bake an additional 40 minutes covered.

Remove the lid and bake a final 10 to 15 minutes until the internal temperature reaches 190-200 degrees F.

Cool completely on a wire rack for at least 2 hours before slicing.