Best on the day, this fudgy chocolate babka is easy to wrap and take with you to share with a friend alongside a good cup of coffee
Heat the oven to 200°C/180°C fan/Gas 6.
Make the filling. Tip the hazelnuts into a baking tray and roast in the bottom of the oven for 4–5 minutes, tossing occasionally, until light golden. Tip onto a chopping board, leave to cool, then roughly chop half the hazelnuts and finely chop the remainder. Set aside.
Place the butter, sugar and chocolate in a pan and melt very slowly over a low heat, stirring until smooth and combined. Remove from the heat and stir in the cocoa powder. Pour into a bowl and leave to cool and thicken slightly.
Meanwhile, make the dough. Tip the flour into the bowl of a stand mixer fitted with the dough hook, add the yeast to one side of the bowl and the sugar and salt to the other side.
Make a well in the centre and pour in the eggs and milk, then mix on slow speed for 2–3 minutes, until firm.
Increase the speed to medium and add the butter, a little at a time. Mix well between each addition, allowing the butter to incorporate before adding more.
Once you have added all the butter, continue kneading with the dough hook on medium speed through the sticky stage, until you have a ball of smooth, silky, shiny dough.
Lightly flour a work surface and roll out the dough to a 40 x 30cm rectangle, with a long edge closest to you.
Spread the cooled chocolate mixture over the dough, leaving a 1cm border all around. Sprinkle all the toasted hazelnuts over the top.
Starting from the long edge closest to you, roll up the dough into a tight spiral, with the seam underneath.
Trim about 2cm off each end to neaten, then turn the roll through 90° clockwise so that a short end is closest to you. Using a large, sharp knife or a pizza cutter, slice lengthways, down through the middle of the dough, cutting it into 2 long pieces.
With the cut-sides facing upwards, gently press the top end of each half together to seal, then lift the right half over the left half, followed by the left half over the right half. Repeat, twisting the dough to make a two-stranded plait, then gently press the bottom ends together to seal.
Carefully lift the loaf into the lined tin and cover with a clean tea towel (or place in a proving bag, if you have one). Leave at room temperature for about 2 hours (or in a proving drawer for 1 hour), until doubled in size.
Fifteen minutes before the end of the proving time, heat the oven to 190°C/170°C fan/Gas 5.
When the babka has proved, bake it for 15 minutes, then reduce the oven temperature to 170°C/150°C fan/Gas 3 and cook for a further 25–30 minutes, until a skewer inserted into the centre comes out clean.
While the babka is baking, make the syrup. Tip the sugar and 100ml water into a small pan, bring to the boil over a medium heat, stirring until the sugar dissolves. Reduce the heat and simmer, without stirring, for 5 minutes, until syrupy. Leave to cool.
When the babka is ready, transfer it in the tin to a wire rack. Brush the cooled syrup over the hot babka, then leave in the tin until warm enough to handle. Turn out onto the wire rack and serve warm or at room temperature.