Step by step recipe:

1. 5 min.

   Prepare the chocolate brioche dough

   Put into a food-mixer bowl: 500 g flour, 15 g yeast, 10 g salt, 20 g brown sugar and 325 g egg.
2. 15 min.
   Knead for 15 minutes.
3. 3 min.
   Then add 100 g Soured dough, 60 g brown sugar and 20 g powdered cocoa.
4. 5 min.
   Knead for a further 5 minutes until well mixed in.
5. 10 min.
   While still kneading, add 200 g butter a little at a time.
6. 1 hour
   Stop kneading once the butter is fully mixed in.
   
   Cover the dough with plastic film and leave to rest at room temperature for 1 hour.
7. 

   Prepare the chocolate-butter square

   While the dough is resting, melt 250 g chocolate with 120 g butter and 50 g brown sugar over a bain-marie.
8. 5 min.
   Mix until smooth. Don't worry if you can feel that there are still some sugar crystals undissolved; this will add texture, so worth keeping.
   
   Tkae the mixture off the bain-marie.
9. 3 min.
   Stand an 8 inch (20 cm) square frame on a sheet of cooking parchment laid on a baking sheet.
10. 2 min.
    Pour the mixture into this mould and leave to cool. Refrigerate overnight.
11. 12 hours
    After the dough has rested for 1 hour, knock back the dough by lifting and dropping hard back into the bowl.
    
    Cover once more with plastic film and refrigerate overnight.
12. 1 min.
    Next day, take the square of chocolate butter out of the fridge and leave at room temperature to soften slightly.
13. 2 min.

    Toast the pecan nuts

    Preheat the oven to 300°F (150°C).
    
    Spread 20 g brown sugar on a baking sheet.
14. 15 min.
    Toast in the oven for 15 minutes.
    
    Leave to cool.
15. 1 min.

    Flake the brioche

    Put the brioche dough on a floured work surface.
16. 3 min.
    Roll out into a large rectangle, about the same width as the square of chocolate butter.
    
    You can trim the sides to neaten the rectangle and use the trimmings to make the leaven or soured dough for your next batch of brioche.
17. 3 min.
    Lay the square of chocolate butter on the dough.
18. 3 min.
    Cover the chocolate with the pecan nuts, either whole (as in the photo) or roughly chopped with a knife.
19. 3 min.
    Fold the dough over to seal in the chocolate butter, then scatter coarse sugar crystals over and press in with the rolling pin to help them stick.
20. 3 min.
    Fold the dough in half over the sugar.
21. 3 min.

    Shape the brioche

    Roll the dough out once more into a rectangle...
22. 3 min.
    ...and roll this up.
23. 3 min.
    Cut the roll into slices about the same thickness as the depth of your tin or mould, either loaf style...
24. 
    ...or special brioche shape.
25. 2 min.
    Use a brush to glaze the top of the brioche with beaten egg yolk.
26. 1 hour 30 min.
    Leave in a warm place to rise for 1 hour 30 minutes.
    
    Before baking, you can scatter a little more coarse sugar over the top if you wish.
27. 40 min.

    Bake the brioche

    Preheat the oven to 390°F (200°C) and bake the brioche for about 40 minutes.
28. 
    Turn out while hot and leave to cool on a wire rack.