Directions:

Make the syrup: Add sugar and water to a pot set over medium-high heat and bring to a boil. Reduce heat and let simmer until sugar dissolves, 2 minutes. Remove from heat and let cool. Cooled syrup can be stored, refrigerated, in an airtight container up to 1 month.

Make dough:
1) Pour milk into a bowl. Activate yeast according to instructions on packet, then add to milk. Add the following ingredients to milk-yeast mixture in this order: flours, eggs, sugar, vanilla, salt and half butter. Using your hands, or an electric mixer with a dough hook set on the lowest speed, blend until combined, 3-4 minutes. Continue blending, increasing speed to medium if using a mixer, and slowly add remaining butter in small chunks until combined, about 4 minutes.

2) Place dough on a work surface lightly dusted with flour and knead by hand until dough starts to feel inflexible, 2 minutes. Shape dough into a square. Place on a tray, cover with plastic wrap and refrigerate 8-12 hours.

Make the babka:
1) Place dough on a work surface lightly dusted with flour. Using a rolling pin, shape dough into a 10-by-28-inch rectangle about 1/5-inch thick. Spread Nutella evenly over dough, then sprinkle dough with chocolate chips. Roll dough tightly like a jelly roll and lay seam-side down. Use a serrated knife to cut roll into thirds, then cut each segment in half lengthwise.

2) Lay two dough pieces on work surface so they form an "X" shape. Twist each end once or twice, so babka resembles a simple braid. Repeat with remaining dough pieces so you have three babka loaves in all. Transfer each to an 8-by-3-inch loaf pan, tucking ends of dough under.

3) Cover pans with a dry towel and let rise until loaves double in volume, about 60 minutes. For best results, place a bowl of warm water at the bottom of an oven that hasn't been turned on, then place loaves inside oven and shut the door.

4) Set oven to convection mode and preheat to 350 degrees. Bake loaves 25 minutes. (If not using convection mode, bake an additional 5 minutes.) Check babkas after 20 minutes. If they are starting to turn dark brown and are still not cooked through, cover with parchment or aluminum foil to prevent tops from burning. Once baked, remove babka from oven and brush immediately with a generous amount of syrup.