Position oven rack in lower third and preheat to 375°F (190°C). Generously butter two 6-ounce ramekins, coating bottom and sides completely. Dust with cocoa powder, tapping out excess. Place on a baking sheet.

Create a double boiler by placing a heatproof bowl over simmering water. Add chopped chocolate and butter, stirring constantly until completely melted and smooth. Remove from heat and let cool slightly.

Whisk egg yolks with 2 tablespoons erythritol until pale and thick. Gradually whisk in the melted chocolate mixture, then add heavy cream, vanilla extract, and salt. Sift in almond flour and fold gently until just combined.

In a spotlessly clean bowl, beat egg whites with cream of tartar using an electric mixer on medium speed until foamy. Gradually add remaining 1 tablespoon erythritol while beating until stiff, glossy peaks form.

Fold one-third of the egg whites into the chocolate base to lighten it. Gently fold in remaining whites in two additions, using a rubber spatula and cutting down through center, across bottom, and up sides, rotating bowl as you fold.

Divide batter between prepared ramekins, filling to within 1/4 inch of rim. Level tops gently with offset spatula. Run your thumb around inside edge to create a shallow moat, which helps soufflés rise evenly.

Bake for 16-18 minutes until soufflés have risen 1-2 inches above rim and tops are set but still jiggle slightly when gently shaken. Do not open oven door during baking.