Special equipment: Three 8-by-4-inch loaf pans and a convection oven (optional)
For the syrup: Put the sugar and 2/3 cup water in a small saucepan over medium-high heat and bring to a boil. Reduce the heat and simmer until the sugar dissolves, about 2 minutes. Remove from the heat and let cool. Cooled syrup can be refrigerated in an air-tight container up to 1 month.
For the dough: Combine the two flours and the dry yeast. Put the milk in the bowl of an electric mixer fitted with the dough hook or in a large bowl. Add in order, the combined flours and yeast, eggs, sugar, vanilla, salt and 4 tablespoons of the butter. Mix on low speed or with your hands until all the ingredients are combined and the mixture is sticky, 3 to 4 minutes. Increase the speed if using a mixer, and slowly add the remaining 4 tablespoons butter in small chunks and mix until the dough is elastic, a bit shiny and very smooth, about 4 minutes.
Put the dough on a work surface lightly dusted with flour and knead by hand until it starts to feel inflexible, about 2 minutes. Shape the dough into a square. Transfer to a baking sheet, cover tightly with plastic wrap and refrigerate until the dough increases in volume by about 30-percent, 8 to 12 hours.
For the babka: Transfer the chilled dough to a work surface lightly dusted with flour. Use a rolling pin to roll the dough into a 1/5-inch-thick rectangle about 10-by-28-inches. Spread the chocolate hazelnut spread evenly over the dough, then sprinkle with the chocolate chips. Roll the dough tightly like a jelly roll and lay seam-side down. Use a serrated knife to cut the roll into thirds, then cut each third in half lengthwise.
Using two dough pieces, lay one across the middle of the other so they form an "X." Twist each end once or twice, so the babka resembles a simple braid. Repeat with the remaining dough pieces so that you have three babka loaves in all. Transfer each babka to an 8-by-4-inch loaf pan, and tuck the ends under.
Cover each pan with a dry towel and let rise until the loaves double in volume, about 1 hour. (For best results, place a bowl of warm water at the bottom of an oven that hasn't been turned on, then place the loaves inside the oven and shut the door.)
Preheat a convection oven to 350 degrees F. (Alternatively, preheat a conventional oven to 350 degrees F.) Bake the loaves until cooked through and the tops are crisp and golden brown, about 25 minutes in a convection oven. If the tops start to turn dark brown and are still not cooked through, cover with parchment or foil to prevent burning. (Cook about 30 minutes in a conventional oven.)
Remove the babkas from the oven and immediately brush them with a generous amount of syrup. Let cool before removing from the pans.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.