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