Step by step recipe:

1. 5 min.

   Prepare the brioche dough

   
   Put into a food-mixer bowl: 250 g flour, 75 g caster sugar, 9 g salt, 10 g yeast, 50 g Soured dough (if possible - it's much better with it), 40 g butter and 350 g egg.
2. 20 min.
   Start the machine and knead for 20 minutes on slow speed.
   
   Note: For the best way to knead, see: A few tips for effective kneading at home.
3. 1 hour
   Gather the dough into a ball and wrap in plastic film. Leave in a warm place for 1 hour.
4. 3 min.
   After this, lift the dough and let it fall sharply back into the bowl once or twice to "knock it back".
5. 12 hours
   Gather the dough roughly into a ball, cover with plastic film and refrigerate overnight.
6. 7 min.

   Make the flaky layers in the dough

   
   Next day, prepare the square of butter, as for french croissants with 150 g butter.
   
   See this video for more details.
7. 3 min.
   Once the square of butter is ready, take the dough out of the fridge and roll it out into a rectangle the same width as the butter, but twice as long.
8. 2 min.
   Place the butter in the centre of the dough...
9. 1 min.
   ...then fold it over...
10. 1 min.
    ...to seal the butter inside.
11. 2 min.
    Press down lightly with the rolling pin.
12. 3 min.
    If necessary, even up the edges by trimming with a knife.
    
    Don't throw away these offcuts; they can become the soured dough for your next batch of viennoiseries (such as brioches, croissants or other types).
13. 3 min.
    Roll out the dough again, keeping the rectangular shape...
14. 2 min.
    ...then fold the ends into the middle...
15. 1 hour
    ...and in half again.
    
    Make a 1/4 turn and repeat this whole operation.
    
    Leave to rest for 1 hour in the fridge.
16. 4 min.

    Add chocolate filling and shape the brioche

    Roll out the brioche dough (now with flaky layers of butter) into a large rectangle as wide as your mould or tin is long, like in this photo.
17. 2 min.
    Moisten the end of your rectangle with beaten egg yolk, using a brush.
18. 4 min.
    Spread 150 g Chocolate ganache over the dough, leaving the end with the beaten egg bare.
19. 2 min.
    Roll the dough up...
20. 
    ...using a ruler to help if you can; it's much easier.
21. 1 min.
    Finish with the seam underneath and brush the top of the roll with beaten egg.
22. 2 min.
    A variation you might like to try (Oriane's idea) - rolled and twisted brioche: split the roll in two along almost its full length...
23. 4 min.
    ...and twist the 2 sections around each other.
24. 1 min.
    Place the brioche in the tin or mould, twisted...
25. 2 hours
    ...or not.
    
    Leave to rise in a warm place for 2 hours.
26. 1 min.

    Bake the flaky chocolate brioche

    
    Preheat the oven to 360°F (180°C).
    
    Give the top of the brioche a final coating of beaten egg...
27. 30 min.
    ...then bake for about 30 minutes.
28. 
    The twisted brioche looks most attractive, very different from the usual style.