In a small pan, heat up the milk, half the sugar and vanilla pod gently until boiling point. Remove from heat immediately and leave to infuse for 15 minutes.
In the meantime, whisk together egg yolks and the remaining sugar until pale and creamy. Sift the flour into the egg yolk mixture and mix well.
Remove the vanilla pod from milk and pour the warm milk over the egg mixture whisking constantly
Pour this mixture into a clean pan and bring to the boil over a very low heat, stirring constantly. Cook for 1-2 minutes very gently. Whisk in the butter and leave to cool for 2 minutes.
For the Dishes:
Preheat the oven to 190°C with the baking tray. Brush the insides of 8 300ml soufflé dishes with softened butter. Coat insides of dish with caster sugar and tip out the excess.
To turn the Crème Pâtissière into the Soufflé Mixture:
Whisk in the cocoa powder, cognac and chocolate pieces into the warm crème pâtissière. Beat egg whites in a clean, dry bowl until firm peaks form. Whisk in sugar to the egg whites gradually to make a stiff glossy mixture.
Whisk half the egg white mixture into the chocolate crème pâtissière to loosen it. Fold in the remainder using a metal spoon. Pour into soufflé dishes and run your thumb around the inside rim to the depth of your nail into the soufflé mixture to help the soufflés rise without sticking.
Put the dishes on the hot baking tray in the oven for 15-18 minutes or until the soufflés are well risen (by at least half the height of the dish) and wobble slightly when tapped.
Use the baking time to make the hot chocolate sauce:
Bring the cream very slowly to boiling point. Immediately remove from the heat. Stir in chocolate and butter.
Check that the soufflé is cooked by inserting a skewer through the side: if the skewer comes out cleanly or slightly moist, you’re good to go! Serve the soufflés immediately with a dollop of vanilla ice cream placed on top and the warm chocolate sauce.