- With the rack in the middle position, preheat the oven to 375°F (190°C). Butter six to eight 1-cup (250 ml) straight-edged ramekins and generously sprinkle with sugar. Refrigerate.
- Place the chocolate in a large bowl. Set aside.
- In a small saucepan off the heat, dissolve the cornstarch in the milk. Bring to a boil and simmer for 30 seconds or until the mixture has thickened, stirring constantly. Pour over the chocolate and let melt for 2 minutes.
- Gently stir the mixture until smooth. With a spatula, stir in the egg yolks. Set aside.
- In another bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add the sugar while beating until stiff peaks form.
- Stir one-third of the meringue into the chocolate base to lighten. Very gently fold in the remaining meringue in two batches using a whisk, maintaining as much volume as possible. Spoon into the ramekins, filling to the top. Run the tip of a finger between the soufflé mixture and ramekin edge to create a groove. Wipe the rims clean. Place the ramekins on a baking sheet.
- Bake for 20 minutes or until the soufflés have risen at least 1 inch (2.5 cm) above the ramekin rim. Serve immediately.
Because the volume of the soufflé mixture can vary, you will be able to fill between six and eight ramekins.