Break up the yeast into the milk and dissolve it. Add the flour, then the eggs, sugar, vanilla, salt, then half of the butter. You can use a stand mixer or knead by hand, for about 7 minutes, until the dough comes together.
Add the rest of the butter, cube by cube, only adding more butter once the previous cube is incorporated. This should take about 3 minutes. Knead the dough into a smooth ball.
Form the dough into a 6x10 inches rectangle, over with plastic wrap or place in a big zip lock bag and refrigerate for at least 4 hours but up to 24 hours.The longer it sits, the better.
Grease 2 loaf pans with butter, non stick spray or use baking parchment.
Lightly flour the work surface and roll the dough into a rectangle measuring about 36 inches by 9 inches.
Spread the Nutella evenly onto the dough, sprinkle the chocolate chips and the chopped walnuts.
Roll the dough tightly, like in the step by step pictures above.Using a knife, cut the roulade in half lengthwise.Then cut it crosswise in half. Make an X with 2 of the pieces, cut-side up. Twist the two halves around each other, like in the pictures. Do the same with the other 2 pieces, and place them in the loaf pans.
Cover the babkas with a towel and set in a warm place until doubled in size, for 1 - 2 or even 3 hours, depending on the temperature in your kitchen.
To make the sugar syrup, combine the sugar and water in a small saucepan over medium heat. Bring to a boil and simmer for 2 minutes until the sugar isdissolved. Set aside to cool.
Bake the babkas in the 350°F / 180°C preheated oven for 25-30 minutes. Every oven bakes differently, so after 20 minutes you can start watching it, so it doesn't burn. It has a lot of chocolate, so it will look pretty dark and caramelized anyway.
Remove from the oven and immediately brush generously with the sugar syrup.
Let the babkas cool completely before taking them out from the loaf pans.
