Heat oven to 400. Butter and sugar six 6-ounce soufflée ramekins. Place on a rimmed baking sheet; set aside.
In a double boiler over medium heat, melt the chocolate until smooth. Remove from the heat and keep warm.In a medium heavy-bottom saucepan combine milk and cornstarch. Stir well with a wooden spoon to combine.
Bring the mixture to a boil over medium heat, stirring continuously, until thick.Remove milk mixture from the heat and stir in warm melted chocolate. Let cool slightly.
Add lightly beaten egg yolks and stir until well combined.In the bowl of a heavy-duty mixer, whip egg whites on medium speed until foamy. Slowly add sugar and increase speed to high. Whip until shiny and stiff peaks form, about 3 minutes.
Using a whisk, lighten the chocolate mixture with about 1/3 of the beaten egg whites. Stir until well-combined.
Using a large rubber spatula, fold in remaining egg whites until just incorporated.Spoon mixture into prepared soufflée ramekins; the mixture should come up to the top of the ramekin.
Transfer filled soufflée ramekins on rimmed baking sheet to oven. Bake until risen, 12 to 15 minutes. Serve immediately.