Preheat the oven to 200°.
Boil milk in a pot, then turn off the flame. Stir in chocolate and mix until it is melted and the mixture is smooth.
When chocolate is at room temp, add yolks one at a time, stirring well. Add cornflour and whisk until mixed in.
In a large bowl, using an electric mixer, beat the egg whites (and powder sugar if used) on medium-high until good solid peaks form (but not super stiff!), say 7 mins.
Fold the egg-white mixture into the chocolate base with a spatula, gently cutting down through the center and lifting up the mix from the bottom of the bowl. Turning the bowl as you go, steadily continue to mix in the egg whites until just combined.
Transfer the mixture to six, 9 cm wide ramekins, fill them to 0.5 cm to the rim, and bake your soufflés until puffed and set, 12 minutes for average sized ones.
Do not open the oven until you are absolutely sure the soufflés are ready; when good to go, serve immediately.