Heat up the milk in a small bowl, in the microwave for a few seconds (10-15). The right temperature of the milk should be between 26°C and 32°C to activate the yeast. Stir the crumbled yeast into the warm milk until dissolved.

In the bowl of a stand mixer fitted with the dough hook, add the flour, sugar, salt, and mix on low speed. Add the eggs, one at a time, the dissolved yeast and keep mixing on low until all the ingredients are well combined. Stop the mixer once to scrape down the sides.

Still on low speed, gradually add the butter, a few cubes at a time. Once all the butter has been added, turn the mixer up to medium speed and continue mixing until the dough is smooth and shiny and has pulled away from the sides of the bowl. This should take 15-25 minutes, no less.

Gently, shape the dough into a ball and transfer to a lightly oiled bowl. Cover with plastic wrap and let rise at room temperature for 1 hour, or until doubled in size.

Punch down the dough, cover again, and refrigerate overnight.

Remove the dough from the refrigerator and transfer it to a lightly floured surface. Punch it down a second time and shape it into a rectangle. At this point, the dough is ready to use, but it can also be wrapped in plastic wrap lightly sprayed with oil and stored in the refrigerator for up to two days or in the freezer for up to one week.

On a bain-marie (double boiler), melt dark chocolate chunks and butter until smooth and shiny. Let it cool a bit, then add cocoa, icing sugar, and cinnamon (if using), and mix. You will get a spreadable paste that looks and smells quite delicious.

Coat a 9-by-4-inch loaf pan with oil or butter, and line the bottom with a rectangle of parchment paper that comes up to the sides (it will be helpful when taking the babka out of the pan).

Take the dough from the fridge (leave the other half in the fridge or freezer for later use). Roll out on a well-floured counter to about a 10-inch width and 12 inches long.

Spread the chocolate mixture evenly over the dough, leaving a ½-inch border all around.

Brush the end farthest away from you with water. Roll the dough up with the filling into a log. Seal the dampened end onto the log. (I followed Smitten Kitchen's instructions and transferred the log in the freezer for 10 minutes. It made it much easier to cut in half).

Trim the last ½-inch off each end of the log. Gently cut the log in half lengthwise and lay them next to each other on the counter, cut sides up. Pinch the top ends gently together. Lift one side over the next, forming a twist, or braid, and trying to keep the cut sides facing out.

Transfer the braid as best as you can into the prepared loaf pan, then nest the trimmed ends of the log in the openings.

Cover with a damp towel and leave to rise another 1 and ½ hours at room temperature.

Heat oven to 375°F (190°C). Remove towel, place loaf pan on the middle rack of the oven. Bake for 25-30 minutes. If it browns too quickly, you can cover it with foil.

While babka is baking, make the syrup: bring sugar and water to a simmer until sugar dissolves. Remove from heat and set aside to cool a little bit.

As soon as the babka leaves the oven, brush the syrup all over it. Let cool about halfway in the pan, then transfer to a cooling rack to cool completely, or eat it when still slightly warm, outstanding!