1. Position rack in center of oven. Heat oven to 400 degrees. Cut a parchment collar about 20 inches long by 6 inches high. Brush the top half of one side with butter. Butter sides only of a 1-quart souffle dish; coat with an even layer of sugar; set aside.

2. In a small heavy-bottomed saucepan, whisk together 1/4 cup sugar and milk. Bring to a boil; remove from heat.

3. In a medium bowl, whisk together 1/4 cup sugar, the flour, and egg yolks. Slowly pour the hot-milk mixture into the egg-yolk mixture, whisking constantly.

4. Transfer milk-and-yolk mixture to saucepan. Whisk over medium-high heat until mixture thickens, about 40 seconds. Make sure to whisk along side of pan to prevent scorching.

5. Remove pastry cream from heat, add chopped chocolate, and whisk until melted. Add cocoa powder; whisk until combined. Transfer to a large mixing bowl.

6. Using an electric mixer fitted with the whisk attachment, combine egg whites and cream of tartar on low speed until frothy. Increase to medium speed until soft peaks form. Gradually add 2 tablespoons sugar. Increase mixer speed to high, and whisk until stiff but not dry peaks form.

7. Whisk chocolate pastry cream to loosen and release steam, add 1/3 of the egg-white mixture, and whisk vigorously until mixture is combined and lightened.

8. Using a rubber spatula, lightly stir remaining egg-white mixture to loosen; this will keep you from overmixing the souffle. Fold remaining egg whites into lightened chocolate cream.

9. Transfer souffle mixture to prepared dish, and smooth top with a spatula. Secure parchment collar around dish with kitchen twine so that the collar extends 3 inches above the dish. Place in oven, and bake for 10 minutes; reduce oven temperature to 350 degrees, and cook for 20 minutes more for a creamy center or 25 minutes more for a slightly drier center.

10. If desired, dust souffle with confectioners' sugar; serve immediately.