Combine half the flour with the yeast in a medium bowl and add the milk and the honey. Mix with a wooden spoon to combine, then use your hands to bring the dough together. Form a ball, and place back into the bowl. Cover with clingfilm and leave to rise in a warm place until doubled in size, about 1-2 hours, depending on the temperature of your kitchen.
Once the dough has risen, add the remaining flour, 40g sugar, 40g butter and the egg. Mix together until it forms a rough dough, then knead it on a lightly floured surface for 5-7 minutes until smooth. It might be sticky at first, but it will become smoother once it has been worked a little bit.
Place the dough back into the bowl, cover with clingfilm and leave to rise until it has doubled in size again.
Shortly before your dough is ready, melt the chocolate with the remaining sugar and butter in a small saucepan over a gentle heat. Preheat the oven to 190°C/gas mark 5. Line a 2lb (roughly 8.5cm x 19cm x 7cm) loaf tin with baking parchment.
Lightly dust your work surface with flour and roll out the dough to a rectangle so that it's roughly 30cm in width and as long as possible in length. The dough should only be 3-4mm thick. Spread the chocolate filling all over the dough, leaving a 1.5cm border around all sides. Sprinkle the toasted hazelnuts all over the filling. Starting with the width closest to you, roll the dough up into a tight roll.
Using a sharp knife, carefully cut the roll down its length in half, and place the two lengths next to each other, cut sides up. Pick up the end of the lengths and start twisting the lengths towards you from the middle. Repeat the twist with the other two ends and pinch the ends together to seal, so that you end up with one long twist. Carefully transfer the twist to the tin. Cover loosely with clingfilm and leave to rise for about 30-45 minutes.
Once risen, sprinkle with demerera sugar, then bake in the oven for 25-30 minutes. Remove from the oven, leave to cool in the tin for 5 minutes, then transfer the loaf to a wire rack and leave to cool completely.