Preheat your oven to 375°F. Generously butter four 6-ounce ramekins, then coat the insides with a thin layer of granulated erythritol, tapping out the excess. This coating helps the souffle climb and rise evenly. Place the ramekins on a baking sheet.
Melt the chopped sugar-free dark chocolate and 3 tablespoons of butter together in a heatproof bowl over a pot of simmering water, stirring until smooth. Remove from heat and let it cool for 3 minutes.
Whisk the egg yolks and 1 tablespoon of the powdered erythritol into the chocolate mixture until smooth and glossy. Stir in the vanilla extract.
In a large, very clean bowl, beat the egg whites with the cream of tartar and salt on medium speed until soft peaks form. Gradually add the remaining 2 tablespoons of powdered erythritol and beat on high until the whites are stiff and glossy but not dry.
Fold one-third of the beaten egg whites into the chocolate mixture vigorously to lighten it. Then fold in the remaining whites in two additions very gently, using slow sweeping motions from the bottom up. Stop as soon as no white streaks remain.
Divide the batter among the prepared ramekins, filling them to about three-quarters full. Run your thumb around the inside rim of each ramekin to create a shallow channel - this helps the souffle rise with a flat top.
Bake for 12 to 14 minutes until the souffles have risen dramatically above the rims and the tops are set but the centers still jiggle slightly. Dust with powdered erythritol and serve immediately - souffles wait for no one.
