1. In the bowl of a stand mixer, mix the yeast with milk and sugar and 1/3 cup (40g) of the flour. Let rest until small bubbles appear and break the surface, about 10 to 15 minutes.
2. With the mixer fitted with the dough hook, on low speed, mix in the 3 ounces butter then the egg and salt. Gradually add the flour until it’s incorporated. Turn the mixer to medium-high speed and knead the dough until smooth, about 5 minutes.
(Note: The original recipe called for an additional 1/4 cup (35g) of flour. When I used that amount, my dough became quite stiff, so I reduced it to the amount here. If your dough seems too soft and is sticking considerably to the sides of the mixer bowl after kneading, add more flour, 1 tablespoon at a time, until it pulls away from the sides. The original recipe called for a total of 330 grams of “strong white wheat flour,” which is about 2 1/4 cups.)
3. Either cover the bowl with a kitchen towel and refrigerate the dough for 6 hours, or overnight, or roll out in the next step.
(Note: I found the dough firm enough to roll right away, but it may be easier to roll if you refrigerate it. And often bread benefits from a leisurely rise, so if you can spare the time, I recommend it.)
4. Butter a 9-inch (23cm) loaf pan and line the bottom and up the sides with a piece of parchment paper overhanging the two long sides, which will help you remove the baked babka later.
5. To make the filling, melt the 3 1/2 ounces of butter in a medium saucepan. Add the sugar and stir until dissolved or almost completely dissolved. (It’s okay if there are grains of sugar visible – they’ll melt later.) Remove from heat and add the chocolate. Let stand 1 minute, then stir until the chocolate is melted and smooth. Stir in the cocoa powder and cinnamon. Set aside.
6. On a lightly floured surface, roll the dough (chilled or at room temperature) to a rectangle 12 x 20-inches (30 x 50cm). If the dough contracts and resists when rolling it, roll it out partially into a rectangle, let it sit 5 to 10 minutes, then continue to roll it out to the final dimensions once it’s relaxed.
7. Spread the chocolate filling over the surface of the rectangle all the way to the edges. Strew the nuts and cookie or brownie bits (if using) over the chocolate. Starting at one of the long ends of the rectangle, roll up the dough tightly so you have a log that’s 20 inches (50cm) long.
8. Using a sharp knife, slice the dough completely in half lengthwise. With the cut sides facing up, overlap the end of one cut half over the other (with the cut sides still facing up), then take the other cut half and fold it over the other, making sure the cut sides are always facing up.
Continue making a rope-like formation (check the photos in the post) overlapping and twisting the two halves of the dough together until the dough into one big twist. Any nuts or filling that have fallen out, toss into the bottom of the loaf pan.
9. Lift the twisted loaf and squidge it into the prepared loaf pan by pushing in on the two ends, so it fits in nicely.
10. Put the loaf pan in a very warm place, such as near a radiator or in an oven that has a pilot light, and let rise for about two hours, until it’s puffy and almost doubled in size.
11. While the dough is rising, make the syrup by bringing the water, sugar, and honey to a boil in a small saucepan. Let boil for 4 minutes, skimming off any foam that rises to the surface with a spoon. Remove from heat and set aside.
12. Fifteen minutes before you bake the babka, preheat the oven to 375ºF (190ºC). Bake the babka on the middle rack of the oven for 30 minutes, or until a skewer inserted into the center (in a part where there is less chocolate filling), comes out clean of dough. There may be some bits of chocolate clinging to it, which are normal.
13. Remove the babka from the oven and spoon or brush the room temperature syrup over the babka and let cool completely before lifting the babka out. Do not try to remove it or slice it while it’s warm, or it will break.
Serve the babka sliced. It will keep for up to 4 days at room temperature or can be frozen for up to two months, if well-wrapped.