Preparing the Baking Dishes

Lightly butter eight ½-cup soufflé dishes or a 1½-quart soufflé dish. Sprinkle with granulated sugar, shaking and rotating the dishes to coat every surface, then pour out any extra sugar. Refrigerate the dishes until ready to use.

Separating the Eggs

Separate the eggs, leaving the whites in a large bowl and the yolks in a small one. Lightly beat the yolks. Set it all aside.

Preparing the Sauce Base

Melt the chocolate in a double boiler over barely simmering water, stirring occasionally, and set aside. In a saucepan, melt the butter, add the flour, and cook over medium heat for 2 to 3 minutes. Add the milk and whisk until the mixture is smooth, then bring to a boil. Reduce the heat and continue to cook for 3 to 4 minutes or until the sauce base is very thick. Remove from the heat, and whisk a little of the warm base mixture into the egg yolks to temper them. Then whisk this mixture into the remaining base. Stir in the melted chocolate, vanilla, and zest, and set aside.

Beating the Egg Whites

In a separate bowl, beat the egg whites with the cream of tartar until soft peaks form (they should still be drooping over, not standing up stiffly, when you lift up the beaters). Sprinkle in the ¼ cup sugar gradually and continue to beat until the whites are stiff but not dry.

Folding the Egg Whites and Finishing the Soufflé

Stir a quarter of the whites into the chocolate mixture to lighten it, and then carefully fold in the remaining whites. Pour the mixture into the prepared soufflé dishes and place on a baking sheet. (You don’t need the baking sheet for a single large soufflé dish.)

Bake for 15 to 17 minutes, until a wooden skewer inserted into the center of the soufflé comes out moist but not gooey. Soufflés will puff and crack before they are done, so don’t worry. One large soufflé will take 35 to 40 minutes.

Remove from the oven, dust with powdered sugar, and garnish each serving with a spoonful or two of fresh raspberry puree, if desired.