1. Preheat oven to 375F (190C) and place the rack in the lowest part of the oven.
2. Brush ramekins with butter, dust with granulated sugar into each dish, turning to coat all sides. pour out the excess. Keep in the fridge until the batter is ready.
3. place chocolate in a large bowl. To a small saucepan pour milk, add cornstarch and bring to a simmer. Cook, stirring, until thick, 1 to 2 minutes. Pour the milk mixture into chocolate and whisk. Add egg yolks and whisk until smooth.
4. whip egg whites with pinch salt until foamy. Gradually add sugar and whip to stiff peaks.
5. Fold one third of the meringue mixture into the chocolate mixture, then fold in the remaining two-thirds.
6. Pour the mixture into the soufflé dishes and bake for 12-15 minutes 7. Dust with powdered sugar and serve immediately.