Chocolate Hazelnut Babka

To make the dough:

1. In a small bowl, sprinkle the yeast over the milk. Stir to combine and allow to rest until frothy, about 10 minutes.
2. In the bowl of a stand mixer fitted with a dough hook, combine the flour, sugar, salt, lemon zest, and vanilla extract.
3. Mix in the eggs and milk with yeast until dough starts to come together.
4. While the mixer is running on low, slowly add the butter, a tablespoon at a time, then continue to mix until fully incorporated and the dough is smooth. It might be a little sticky. If the dough is too crumbly, add a little more milk.
5. Lightly grease a large bowl with oil, then add the dough, turning to coat. Cover and refrigerate 8 hours to overnight. The dough will puff, but probably not double.

To assemble:

1. Remove the bowl from the refrigerator and allow to rest at room temperature, covered, for 1 hour.
2. Grease 2 9x4 inch (23x10 centimeter) loaf pans with butter or oil. Line the bottom and two long sides with parchment.
3. Divide the dough in half.
4. On a lightly floured surface, roll one half of the dough into a thin rectangle/square 10 inches wide (25 centimeters) and 10-12 inches (25-30 centimeters) long.
5. Spread half of the Nutella over the dough, leaving a 1/2 inch (1.25 centimeter) border around the edges.
6. Sprinkle evenly with half of the chopped chocolate. Wet the 1/2 inch (1.25 centimeter) edge furthest from you with water.
7. Starting with the edge closest to you, tightly roll up the dough, sealing the dampened edge.
8. If desired, place the roll in the freezer for 10 minutes to harden the chocolate slightly and allow the dough to be worked with more cleanly.
9. Slice off 1/2 inch (1.25 centimeters) off each end of the dough and discard.
10. Slice the rope in half lengthwise and place on the work surface cut side up. Twist together the ropes, cut side facing out, end to end. You can also cross the ropes in the middle, then twist them to the ends starting from the middle.
11. Place the twisted loaf in the prepared pan and cover with a damp cloth. Repeat with remaining dough and filling and transfer to the second loaf pan. Allow to rise at room temperature until puffed, 1-2 hours.

To finish:

1. Preheat oven to 350˚F (180˚C). Bake the prepared loaves until deeply golden and cooked through, about 40-50 minutes.
2. While the loaves are in the oven, prepare the syrup.
3. In a small pot, combine the water and sugar over medium heat. Simmer until the sugar dissolves completely. Remove from heat and allow to cool slightly.
4. Removed the baked babkas from the oven and immediately brush with the sugar syrup.
5. Allow to cool for a few minutes before removing from the pans. Cool to room temperature before slicing and serving.