Brush 8 ramekins (8cm in width, 4-5cm in height) with the softened butter in upward strokes, from the base to the rim. Chill the dishes for 5 minutes, then as an insurance policy so the soufflé doesn't stick to the dish, apply a second coat as before.
Sprinkle a little grated chocolate into each dish, tilting and rolling the dishes around to line them with an even layer of chocolate.
For the crème pâtissière, slake the corn flour with a little of the milk, then gradually add the rest of the milk until you have a smooth mixture. Pour into a small pan, and slowly bring to the boil, stirring continuously. Boil for 30 seconds then off the heat, add the chocolate and whisk until smooth. Beat in the egg yolks until smooth before transferring to a bowl to cool.
Egg white mixture
In an artisan mixer, whisk the egg whites to soft peaks. Gradually start to add in the sugar, whisking well in between each addition. Once all the sugar has been added, continue to whisk until you have a thick and glossy mixture.
Add one third of the egg whites into the chocolate crème patisserie mixture and mix well. Now add in the remaining egg white and gently fold into the mix. Do not over work the mixture as it will become too hard and stiff.
Divide the mixture between the ramekins. Level the surface with a palate knife then bake for 6-8 minutes, or until well risen and with a slightly wobbly centre. Remove from the oven and dust with the remaining cocoa powder. Serve immediately with a scoop of mint chocolate chip ice cream.