Preheat the oven to 180˚C. Use half the butter to grease the insides of the soufflé dishes. Place in the fridge until the butter has set hard. Grease the dishes again using the remaining butter, this time also greasing the top edge. Sprinkle the insides with caster sugar to form a light coating, tipping out any excess, then sprinkle with cocoa powder to coat, also tipping out any excess. Set the prepared dishes aside in a cool place.
Sift the 20g cocoa powder and flour together into a bowl. In another bowl, beat the egg yolks with 80g caster sugar until thick and creamy. Stir in the sifted mixture and set aside.
Bring the milk to the boil over a medium heat, then pour over the cocoa mixture and stir until well blended. Tip the mixture back into the pan, place over a low heat and bring gently to the boil. Simmer for 5 minutes, stirring continuously, until the mixture thickens and forms a smooth custard. Set aside.
Melt the chocolate in a heatproof bowl set over a pan of gently simmering water, making sure the bottom of the bowl I not in contact with the water. Stir the melted chocolate into the custard mixture until well blended. Set aside and leave to cool.
Whisk the egg whites until they form soft peaks. Fold in the remaining caster sugar and whisk again until the mixture forms soft peaks. Stir 2 tablespoons into the cooled chocolate custard mixture, then carefully fold in the remainder until evenly blended. Do not overwork.
Divide the mixture between the prepared soufflé dishes, placing about 2 tablespoons in each. Level off the tops and run a finger around the edge of each to push the mixture away from the sides. Bake in the preheated oven for 12-13 minutes, until well risen.
Meanwhile make the sauce. Place the milk, cream and sugar in a saucepan over a medium heat until just starting to boil. Remove from the heat and stir in the chocolate until melted and evenly mixed. Return the pan to a low heat, add the butter and stir until melted and the sauce is smooth and evenly blended.
Serve the cooked soufflés immediately, with the hot chocolate sauce handed separately in a jug.