Preheat the oven to 400° with the rack in the center of the oven.
In a small bowl, mix together 1 Tablespoon of the sugar with the cornstarch. Whisk in the egg yolks and set aside.
For the soufflé base, place the milk in a small sauce pan over medium heat to warm. In a separate small saucepan, melt the butter over medium heat. Whisk in the flour and a pinch of kosher salt into the butter mixture. Whisk for about 30 seconds.
Remove from heat. Whisk in 1/3 of milk to form a smooth mixture without lumps. Whisk in another 1/3 of the milk and then the final third. Return to medium heat and whisk, bringing to a simmer. Simmer for 30 seconds then remove the pan from the heat. Immediately whisk in the chocolate to melt. Whisk in the egg yolk mixture. Place the base in a large mixing bowl.
Brush your baking dish (or dishegenerously with softened butter. At the end, brush the butter vertically around the sides of the pan, so that the soufflé will rise along the lines. Coat the pan withgranulated sugar.
In a large bowl, or the bowl of your stand mixer, whisk the egg whites until they foam. Whisk in 1/3 of the remaining sugar until the whites begin to hold a form. Add another 1/3 of the sugar and continue to whisk until the whites hold a bit more of a shape. Add the final 1/3 of the sugar and continue whisking and add the remaining sugar until the whites are shiny and hold a firmer shape. Do not over whisk the whites. They should be glossy and hold a shape but over-whisking can cause them to break down.
Gently stir about 1/3 of the egg whites into the chocolate mixture to combine and lighten the batter. Fold the remaining whites in the batter.
Spoon the soufflé into the prepared dish to reach the almost top of the pan. (Any extra can be placed in a ramekin and baked alongside the pan). Bake the soufflé anywhere from 15 minutes (for individual ramekinto 30 minutes (for one larger pan), depending on the size of the baking dish, or until it has risen and the top is set (check with a skewer or about 160° with instant read thermometer). *Mine took close to 30 minutes for this one large-ish souffle.
Serve immediately, with whipped cream, creme anglaise or vanilla ice cream and a few raspberries, if desired. Serves 2 - 4.
Be sure to read the Cook's Notes above the Recipe Card for more tips on making this recipe.