Prepare the filling: melt the chocolate and butter, then add the salt, vanilla and sugar to taste and ground hazelnuts. As far as I'm concerned, I don't add sugar as the chocolate is already sweet enough. Mix thoroughly. If the filling is too thin, refrigerate for about 30 minutes to make it easier to spread. After the first rising, roll out the butter brioche dough into a square about 5 mm thick, cover
with the filling, roll up into a sausage shape and cut in half lengthways. Twist and join the ends in a circle. The result will be a delicious chocolate and nut babka.
For the egg glaze mix all the ingredients together to form a runny/smooth glaze. Place the dough on a sheet of baking paper, brush with the egg glaze, cover with cling film and leave to rise until doubled in size. Brush again with the egg glaze. Bake in a preheated oven at 190°C/375°F/gas 5 until the babka has a nice yellow colour. Then lower the temperature to 170°C/340°F/gas 3½ and
continue cooking until golden brown.
Instead of the chocolate hazelnut filling, you can also use a thick jam. A homemade plum jam goes wonderfully with the buttery crumb. Or you can replace the hazelnuts with ground walnuts, almonds, etc.