Combine the flour, sugar, yeast and zest in the bottom of the bowl of a stand mixer. Add eggs and 1/2 cup water, mixing with the dough hook until it comes together; this may take a couple minutes. It’s okay if it’s on the dry side, but if it doesn’t come together at all, add extra water, 1 tablespoon at a time, until the dough forms a mass. With the mixer on low, add the salt, then the butter, a spoonful at a time, mixing until it’s incorporated into the dough. Then, mix on medium speed for 10 minutes until dough is completely smooth; you’ll need to scrape the bowl down a few times. I usually found that after 10 minutes, the dough began to pull away from the sides of the bowl. If it doesn’t, you can add 1 tablespoon extra flour to help this along.

Coat a large bowl with oil (or scrape the dough out onto a counter and oil this one) and place dough inside, cover with plastic and refrigerate. Leave in fridge for at least half a day, preferably overnight.

Melt butter and chocolate together until smooth. Stir in powdered sugar and cocoa; mixture should form a spreadable paste. Add cinnamon, if desired.

Coat two 9-by-4-inch (2 1/4 or 1kg) loaf pans with oil or butter, and line the bottom of each with a rectangle of parchment paper. Take half of dough from fridge (leave the other half chilled). Roll out on a well-floured counter to about a 10-inch width and as long in length as you can when rolling it thin, likely 10 to 12 inches.

Spread half of chocolate mixture evenly over the dough, leaving a 1/2-inch border all around. Brush the end farthest away from you with water. Roll the dough up with the filling into a long, tight cigar. Seal the dampened end onto the log. Transfer the log to a lightly floured baking tray in the freezer for 10 to 15 minutes to make it easier to cut cleanly in half. Repeat with second dough.

Trim last 1/2-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 and trying to keep the cut sides facing out.

Transfer the twist as best as you can into the prepared loaf pan. The dough will fill in any gaps by the time it’s done rising and baking.

Cover with a damp tea towel and leave to rise another 1 to 1 1/2 hours at room temperature. Repeat process with second loaf.

Heat oven to 375°F (190°C). Remove towels, place each loaf on the middle rack of your oven. Bake for 30 minutes, but there’s no harm in checking for doneness at 25 minutes.

Bring sugar and water to a simmer until sugar dissolves. Remove from heat and set aside to cool somewhat. As soon as the babkas leave the oven, brush the syrup all over each. Let cool about halfway in pan, then transfer to a cooling rack to cool the rest of the way before eating.