In a small jar, combine 25g starter, 40g flour, and 40g water. Mix until no dry flour remains.

Cover loosely and let ripen at room temperature (70-75°F) for 8-12 hours, or until doubled, bubbly throughout, and domed on top.

The levain is ready when it smells mildly sweet and tangy, not sharp or alcoholic. It should pass the float test.

In the bowl of a stand mixer fitted with the dough hook, combine all-purpose flour, milk, eggs, sugar, and all of the ripe levain.

Mix on low speed (setting 2) for 2-3 minutes until a shaggy dough forms and no dry flour remains.

Cover the bowl and let rest for 20-30 minutes. This hydrates the flour and makes the dough easier to work with.

Sprinkle the salt over the dough. Mix on medium-low speed (setting 3) for 2 minutes until incorporated.

With the mixer running on medium-low, add the softened butter one or two pieces at a time. Wait until each addition is mostly incorporated before adding more. This takes 8-10 minutes total.

Continue mixing on medium speed (setting 4) for another 5-8 minutes until the dough pulls away cleanly from the sides, is smooth, shiny, and elastic, and passes the windowpane test.

Transfer the dough to a lightly greased bowl or container.

Perform stretch and folds: wet your hands, grab one side of the dough, stretch it up, and fold it over to the opposite side. Rotate the bowl 90° and repeat 4 times total. Cover and rest 30-45 minutes.

Repeat the stretch and fold process. Cover and rest undisturbed for 1-2 more hours at room temperature.

The dough is ready when it has increased by about 50% (not doubled), looks puffy and slightly domed, and has small bubbles visible on the surface.

Cover the bowl tightly with plastic wrap.

Refrigerate for at least 8 hours or up to 24 hours.

Place a heatproof bowl over a pot of barely simmering water. The bowl should not touch the water.

Add the chopped chocolate and butter. Stir occasionally until melted and smooth.

Remove from heat. Whisk in the cocoa powder, powdered sugar, cinnamon (if using), and salt until completely smooth.

Let the filling cool to room temperature, about 30-45 minutes. It should be spreadable like soft peanut butter, not runny.

Grease a 9x5-inch loaf pan with butter or cooking spray. Line the bottom with a strip of parchment paper for easy removal.

Remove the dough from the refrigerator. On a lightly floured surface, roll into a rectangle approximately 12x16 inches (30x40 cm), with the long side facing you. The dough should be about ¼ inch thick.

Spread the chocolate filling evenly over the dough, leaving a ½-inch border on all sides.

Starting from the long side closest to you, roll the dough up tightly into a log. Pinch the seam to seal.

Place the log seam-side down on the counter. Using a sharp knife or bench scraper, slice the log in half lengthwise, exposing all the layers.

Lay the two halves side by side, cut sides facing up.

Pinch the top ends together firmly.

Cross the right half over the left, keeping the cut sides facing up. Then cross the left over the right. Continue alternating until you reach the end.

Pinch the bottom ends together and tuck underneath.

Carefully transfer the braided loaf to the prepared pan, gently pressing it down to fit.

Cover the pan loosely with a damp towel or plastic wrap.

Let proof at room temperature (75-78°F) for 3-4 hours.

The dough is ready when it has risen to about ½ inch below the rim of the pan, looks puffy, and jiggles slightly when you shake the pan. A gentle poke leaves an indent that slowly springs back.

During the last 30 minutes of proofing, preheat your oven to 350°F (175°C).

If using egg wash, whisk together the egg and milk. Gently brush the top of the babka.

Bake for 20 minutes.

If the top is browning too quickly, tent loosely with aluminum foil.

Bake for another 20-25 minutes until deep golden brown and the internal temperature reaches 185-190°F (85-88°C).

The babka should sound hollow when tapped. Total baking time: 40-45 minutes.