If you're baking the soufflés right away, position a rack in the lower third of the oven and preheat to 375 degrees.
Butter the 6oz ramekins and dust the insides with sugar.
Place the chocolate, butter and milk in a large heatproof bowl (preferably stainless steel) in a large skillet of barely simmering water.
Stir until the chocolate is melted and the mixture is smooth.
Remove the bowl from the water bath and whisk in the egg yolks.
(Don't worry if the mixture stiffens slightly or is less than perfectly smooth at this point.) Set aside.
In a medium, dry bowl, beat the egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form when the beaters are lifted.
Gradually sprinkle in the 1/3 cup sugar and beat at high speed until the whites are stiff but not dry.
Fold 1/4 of the egg whites into the chocolate mixture to lighten it, then fold in the remaining egg whites.
Divide the mixture evenly among the prepared ramekins, filling each 3/4 full.
(The soufflés can be prepared to this point, covered, and refrigerated for up to 2 days. Bake directly from the refrigerator.) Place the soufflés on a cookie sheet.
Bake until they rise and crack on top and a wooden skewer plunged into the center emerges very moist and gooey (but the centers should not be completely liquid), 14 to 16 minutes, perhaps a minute or so longer if the soufflés have been refrigerated.
Meanwhile, make the topping: Beat the cream with the vanilla and sugar until it holds a soft shape (or stiffer, if you like it that way).
Transfer to a serving bowl and refrigerate until ready serve.
When they are done, remove the soufflés from the oven and serve immediately, with a little powdered sugar sifted over the top, if you like.
Pass the whipped topping separately.
NOTES: You can substitute a lower-percentage bittersweet or semisweet chocolate if you prefer a sweeter, less intense chocolate flavor; or reduce the sugar to 1/4 cup to partially compensate for the sweeter chocolate, if desired.
There is no need to make other changes in the recipe.
After you have buttered the ramekins, the easiest way to dust sugar on the inside is to put the 2 tablespoons of sugar in one of the buttered ramekins.
Tilt and hold that ramekin over another as if you were going to transfer the sugar.
Rotate the ramekin containing the sugar, allowing the sugar to coat the sides as you slowly pour the sugar into the other ramekin.
Room temperature eggs are best because a cold egg might cause the chocolate to seize, which means it gets too stiff to work with.
If you do not have eggs at room temperature, you can hold the egg yolks in a mixing bowl over medium heat (hold the bowl over but not on the burner) to warm them up.
Underbeating egg whites is always better than overbeating them.
You want to beat the whites until they're no longer yellow and translucent.
To get a bigger rise, fill the ramekins higher than suggested.
This will mean that you will have one or two fewer soufflés.
Don't worry about underbaking or overbaking this recipe- The souffles will still taste good.