For the souffle: grind chocolate finely. Melt butter in a saucepan, stir in flour and cook until light brown. Add milk and bring to a boil, stirring. Add chocolate and 2/3 of sugar, remove pan from heat. Whisk egg yolks and add to chocolate mixture. Beat egg whites until very stiff, add remaining sugar gradually. Add 3 tablespoons of egg whites to chocolate mixture, add the rest gradually. Pour batter into buttered and sprinkled with sugar ramekins and bake in preheated oven at 175°C (approximately 350°F) doe about 30 minutes.
Sprinkle with powdered sugar and serve.