Several hours before making the dough, feed your sourdough starter so you have at least 150 grams of active starter for the recipe, plus a little left over to maintain it. For example, combine 35 grams starter, 70 grams water, and 70 grams bread flour. Stir well, mark the level on the jar, and leave it at room temperature until it has doubled in size and is bubbly and active.
Let the starter rise until it has at least doubled and is bubbly and puffy, about 4 to 8 hours depending on your kitchen. Measure out 150 grams of active starter for the dough. Keep the rest to feed for your next bake.
Make the Dough
In a large bowl, combine the active starter and water. Stir to dissolve.
Add the brown sugar, granulated sugar, cocoa powder, espresso powder, bread flour, and salt. Mix until no dry flour remains. Cover and rest 30 minutes.
Stretch and Folds
First stretch and fold. With a damp hand, lift one side of the dough until you feel resistance, then fold it over the center. Rotate the bowl a quarter turn and repeat around all four sides. Cover and rest 30 minutes.
Second stretch and fold. Repeat around all four sides. Cover and rest 30 minutes.
Lamination
Laminate in the chocolate chips. Gently stretch the dough on a lightly dampened surface into a rectangle. Stop where the dough resists. Scatter the chocolate chips evenly over the surface. Fold the dough in thirds, then fold again into a package. Return to the bowl.
Rest
Rest 30 minutes, then coil fold. Slide damp hands under the center of the dough and lift so the ends tuck underneath. Rotate and repeat from the other direction. That is one set. Rest 30 minutes. If the dough still feels loose, do one more coil fold set.
Bulk ferment undisturbed until the dough has risen 50 to 75 percent, looks puffed and aerated, has bubbles along the sides, and jiggles when you move the bowl. This dough ferments slower than a plain loaf because of the sugar and cocoa.
Shape and Cold Ferment
Shape into a boule, building surface tension without pressing out the gas. Place seam-side up in a floured banneton. Stitch the seam if needed.
Refrigerate overnight, 10 to 16 hours.
Bake
Preheat the oven and a Dutch oven at 450°F for 30 minutes. Turn the dough onto parchment, score it, and lower into the Dutch oven.
Bake covered at 450°F for 30 minutes. Uncover, reduce to 425°F, and bake about 15 minutes more.
The loaf is done when the center reads 205 to 208°F on a thermometer. Crust color will not tell you.
Cool on a wire rack for a full 2 hours before slicing.
Notes
Use an active starter. Feed it 6 to 8 hours before mixing.
Weigh everything. Cocoa and sugar throw off the ratios if you use cups.
Do not add more cocoa. It slows the rise and makes the dough dense.
Fold the chips in gently so you do not tear the dough.
This dough ferments slower than plain sourdough. Wait for the puffy, jiggly rise.
Bake to temperature. Aim for 205 to 208°F in the center.
Cool a full 2 hours. The chips stay molten inside a warm loaf.
Store cut-side down the first day, then wrap in a towel. Slice and freeze.