Combine one tablespoon of sugar, the yeast and milk in a small bowl. Let it sit for 10-15 minutes until it begins to foam and bubble.
In your stand mixer fitted with the hook attachment, mix your remaining sugar, flour and salt. Add the egg, egg yolks, vanilla and yeast mixture. Mix it on low for 2-3 minutes, it will take some time for everything to be combined. Increase the speed to medium and mix for 10 minutes. The dough will start off quite dry but it'll come together and become smooth.
Reduce the speed to low and add the butter a piece at a time. Make sure each piece of butter is fully incorporated before adding the next. It might look greasy but it will come together.
Once all the butter has been added, increase the speed to medium and mix for 5 minutes. The dough should become very soft. Take the dough out and oil the bowl. Pop the dough back in and cover in plastic wrap. Let it right for 45 minutes or until doubled in size in a warm place.
Take the dough out and gently press it out into a rectangle, measuring around 8x6 inches. Cover completely in plastic wrap and place in the fridge for at least an hour, or up to overnight. The longer you leave it the more flavor you'll have.
Roll out your chilled dough on a floured surface until it is 10x13 inches. Use a pastry brush or offset spatula to spread out the Nutella. If your Nutella is too stiff then microwave it for 30 seconds. Sprinkle over three quarters of your hazelnuts (you'll keep the rest to go on top of the babka!).
Starting on one long side, tightly roll it up like you would with cinnamon rolls. Carefully lift the loaf and cover it in plastic wrap, then place in the fridge for 30-40 minutes. Chilling it will help the Nutella not seep out.
Grease and line a 9x5 inch loaf pan.
Take your loaf out the fridge and remove the plastic wrap. With the seam side down, use a serrated knife to cut it in half lengthways. Place one log over the side, with the filling side up, to form an X. Twist the upper half twice and then the lower half twice. Carefully place it in your prepared loaf pans, tucking the ends under. Sprinkle over your extra hazelnuts.
Cover the pan in plastic wrap and let it rise for another 30-60 minutes. You'll know it's ready when you gently press the dough and it bounces back but leaves a slight indentation.
Preheat your oven to 350°F/175°C. Bake the babka for 45-50 minutes until it is golden brown. While it's baking make your syrup. Place the water and sugar together in a small saucepan and heat over a medium heat until the sugar has dissolved. Let it cool.
Take the babka out of the oven and brush it over with the syrup. Let it cool for at least one hour.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘US CUSTOMARY’ above the list of ingredients. Ingredients: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your frosting much creamier and richer.Storage: Keep wrapped up for up to 2 days. You can freeze the babka by slicing the babka, wrapping in plastic wrapper, and freezing for up to 3 months.