Make the dough: Combine the flour, sugar, yeast and zest in the bottom of the bowl of a stand mixer. Add eggs and ½ cup water, mixing with the dough hook until it comes together.

With the mixer on low, add the salt, then the butter, a spoonful at a time, mixing until it’s incorporated into the dough. Mix on medium speed for 10 minutes until dough is completely smooth.

Coat a large bowl with oil and place dough inside, cover with plastic and refrigerate for at least half a day, preferably overnight.

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

Assemble loaves: Coat two 9-by-4-inch loaf pans with oil or butter, and line the bottom of each with parchment paper. Roll out half of dough to about a 10-inch width and as long as possible.

Spread half of chocolate mixture evenly over the dough, leaving a ½-inch border. Brush the end farthest away from you with water. Roll the dough up with the filling into a long, tight cigar.

Trim last ½-inch off each end of log. Gently cut the log in half lengthwise and lay them next to each other, cut sides up. Pinch the top ends gently together and twist.

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

Bake: Heat oven to 375°F (190°C). Bake for 30 minutes, checking for doneness at 25 minutes.

Make syrup: Bring sugar and water to a simmer until sugar dissolves. Brush the syrup all over each babka as soon as they leave the oven.