Make the bloom. Warm the milk until just warm to the touch, then combine it with the melted butter in a small bowl or liquid measuring cup. Whisk in the Dutch process cocoa powder until fully incorporated and smooth with no dry streaks remaining. Set aside and allow to cool to room temperature before using. Do not add the bloom to the dough while it is still warm.

In the bowl of your stand mixer, combine the active sourdough starter, room temperature Greek yogurt, egg, vanilla extract, and the bloom. The bloom can be slightly warm when added but should not be hot. Whisk together until smooth and fully combined.

Add the bread flour, brown sugar, salt, and baking powder on top of the wet ingredients. Mix on low speed with the dough hook until a shaggy dough forms with no dry flour remaining. Increase to medium speed and mix for 5 to 6 minutes until the dough is smooth, elastic, and pulls away cleanly from the sides of the bowl. The dough will be slightly tacky but should not be sticky.

In the last 1 to 2 minutes of mixing only, add the mini chocolate chips and continue mixing just until they are evenly distributed throughout the dough. Adding the chips at this stage ensures even distribution in every slice without clustering or sinking during fermentation.

Transfer the dough to a lightly oiled bowl and cover tightly. Allow the dough to rest for 1 hour.

After the 1 hour rest, perform one set of stretch and folds. To do this, wet your hand, grab one side of the dough, stretch it upward, and fold it over the center. Rotate the bowl 90 degrees and repeat until you have gone all the way around. The dough will feel noticeably stronger after this set.

Bulk ferment the dough at room temperature until it feels noticeably lighter, airier, and puffy when pressed gently. In a kitchen at 72 degrees Fahrenheit this takes approximately 6 hours. At 70 degrees expect closer to 8 hours. The cocoa tannins and brown sugar slow fermentation, so trust the feel of the dough over the clock. You may not see significant surface bubbles because the dark color of the dough obscures them. This is normal.

If mixing before bed, allow the dough to bulk ferment at room temperature overnight. Shape in the morning once it feels light and airy.

When bulk fermentation is complete, turn the dough out onto a lightly dusted surface. Dust your rolling pin lightly as well. Do not flour the dough itself. Roll the dough into a rough 12 by 9 inch rectangle, working gently to preserve some of the gas without fully deflating it. Use a bench scraper to help move the dough if needed.

Starting at one of the 9 inch ends, begin rolling the dough toward you. Apply gentle downward pressure with each pass and pull the roll slightly toward you to create tension. You are aiming for a tight, taut log rather than a loose roll. Once you reach the end, pinch the seam firmly closed and place the log seam side down into a greased USA Pan 9x5 loaf pan.

Cover the pan loosely and allow the dough to proof at room temperature until it rises at least 1 inch above the rim of the pan. Use the poke test to confirm readiness. Press one finger gently into the surface of the dough. If it springs back immediately with no impression the dough needs more time. If it springs back slowly and leaves a faint impression the dough is ready to bake. If the impression stays without springing back the dough is overproofed. Note that even when the dough appears to be well above the rim it can still be underproofed, so rely on the poke test over the visual.

If your kitchen is running cool, place the pan in your oven with only the oven light on for up to 1 hour to give the proof a gentle boost.

Preheat your oven to 375 degrees Fahrenheit. For extra oven spring and a softer top crust, place an inverted baking sheet over the loaf pan for the first 10 to 15 minutes of baking. Make sure there is enough clearance between the baking sheet and the dome of the loaf before covering. This step is optional but recommended.

Bake for approximately 50 to 55 minutes total, tenting loosely with aluminum foil after 35 minutes if the top is darkening too quickly. The dark color of this loaf will make it look finished well before it is done. Always use an instant read thermometer and pull the loaf when the internal temperature reaches 200 degrees Fahrenheit.

The moment the loaf comes out of the oven, brush the top generously with melted butter using a pastry brush. This step is essential for achieving a soft, tender crust. Do not skip it and do not wait.

Allow the loaf to rest in the pan for 5 minutes, then transfer to a wire rack with airflow on all sides. Cool completely for a minimum of 2 hours before slicing. The crumb continues to set during this time and cutting too early will result in a gummy interior.