Place the sifted flour and yeast in a bowl and mix, then add sugar, vanilla, salt, eggs and water to the center and start kneading.
Then add the butter in several stages, one piece at a time, without adding more until the previous one has been completely incorporated.
In the end you should obtain a homogeneous and elastic dough, well combined.
Transfer to a lightly buttered bowl, cover with cling film and leave to rise in the fridge overnight.
Prepare the chocolate cream: melt butter and chocolate in a bain-marie, then incorporate cocoa and icing sugar, working very well with the whisk to eliminate any lumps, then leave to cool: you should obtain a super chocolatey and rather dense spreadable cream.
Coarsely chop the hazelnuts.
Take the dough and roll it out on a lightly floured work surface to form a rectangle measuring approximately 30×40 cm.
Spread the entire surface with the cream, leaving only 1-2 cm free along the edges, sprinkle with hazelnuts and sugar and roll up starting from the long side.
With a sharp knife, cut the roll in half lengthwise, then intertwine the 2 halves together, trying to keep the cut parts facing upwards.
Place the brioche in the mold and let it rise again until doubled in size in a warm place, then cook for about 25 minutes or until lightly golden in a fan oven preheated to 180°C.
Meanwhile, prepare a syrup by pouring water and sugar into a saucepan and bringing to the boil, then leave to cool.
Brush with the syrup until it is all used, then leave to cool completely.
The babka is ready, all you have to do is serve it and enjoy it.
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