Mix the warm milk & yeast in a bowl. Leave to stand for a few minutes.
Sift the 1oz of flour into the milk & yeast mix, add the 1 tsp of sugar & mix well. Leave to stand in a warm place for 10 mins or until frothy (will look a little like mousse!).
While the starter is standing, sift the flour & salt into a mixing bowl. Add the vanilla sugar then rub in the butter. ( rub in method - rub the butter into the flour to create 'bread crumbs' - like you do with short crust pastry)).
When the starter is frothy, whisk in the 2 eggs. When the eggs are fully incorporated, pour the mixture into mixing bowl & stir the ingredients until they come together to form a ball.
Remove the ball of dough & knead on a lightly floured surface for a minute or until no longer sticky.
Place the dough on a lightly floured dish or tray, cover & let rest in a warm place for around 90 minutes or until roughly doubled in size.
When doubled in size, knead the dough again for another minute - just to knock the air out a bit.
Dust a work surface with flour then divide the dough into 12 equal pieces ( cut dough into quarters then cut each quarter into thirds). Place a square of chocloate in the centre of each piece of dough then form into a ball.
Lightly grease a baking tin then place the balls inside - 3 in the middle of the tin & 9 around the edges - make sure there are spaces in between.
Place in a preheated oven at 230 c (450 F) for around 18 minutes. Start checking the brioche at 15 minutes, remove from oven when pale gold all over.
Remove from tin & allow to cool.Divide the icing sugar into 2 bowls. Add the cocoa powder to one & add a tiny bit of hot water at a time to each, until you have a thick icing that you drizzle over the brioche. Slowly drizzle the icing over the brioch until it is almost covered.