Mix the flour, salt, sugar and yeast in a bowl. Add the butter, milk and egg, mix and knead to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 1 1/2 hrs. until doubled in size.
Place the chocolate and butter in a thin-sided bowl, suspend the bowl over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Stir in the nuts, cocoa powder and vanilla, leave to cool.
On a lightly floured surface, roll out the dough into a rectangle (approx. 40 x 25 cm), cover with the chocolate mixture, roll up from the long edge. Cut the roll in half lengthwise, braid the strands with the cut sides facing up, place in the prepared cake tin.
Approx. 40 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tin and leave to cool completely on a rack.