01 - Preheat oven to 400°F. Generously butter four 6.75-ounce ramekins and dust with sugar, tapping out any excess.
02 - Melt dark chocolate and butter together in a heatproof bowl set over simmering water. Stir until smooth, then let cool slightly.
03 - Beat egg yolks with 2 tablespoons sugar until pale and thick. Fold in the melted chocolate mixture.
04 - Whisk 3 egg whites with salt and cream of tartar (if using) until soft peaks form. Gradually add 1 tablespoon sugar and continue whisking to stiff peaks.
05 - Gently fold one-third of the egg whites into the chocolate mixture to loosen. Carefully fold in the remaining whites until just combined.
06 - Spoon mixture into prepared ramekins, filling almost to the top. Run your thumb around the inside edge for an even rise.
07 - Place ramekins on a baking sheet and bake for 12-15 minutes until well risen and just set but still slightly jiggly in the center.
08 - Heat milk, cream, and vanilla in a saucepan until just simmering. Whisk egg yolks and sugar until pale. Gradually pour hot milk mixture into yolks, whisking constantly. Return to saucepan and cook over low heat, stirring constantly, until thickened enough to coat the back of a spoon. Strain and keep warm.
09 - Serve soufflés immediately, dusted with powdered sugar and accompanied by warm crème anglaise.