01 - Preheat oven to 400°F. Generously butter four 6.5 oz ramekins and dust with sugar, tapping out excess.
02 - Melt dark chocolate and butter in heatproof bowl over simmering water. Stir until smooth, then cool slightly.
03 - Beat egg yolks with sugar until pale and thick. Fold melted chocolate mixture into yolks.
04 - Whisk egg whites with salt and cream of tartar until soft peaks form. Gradually add remaining sugar and whisk to stiff peaks.
05 - Fold one-third of egg whites into chocolate to lighten. Gently fold remaining whites until just combined.
06 - Spoon mixture into prepared ramekins, filling almost to top. Run thumb around inside rim for even rise.
07 - Place ramekins on baking sheet. Bake 12-15 minutes until risen and set but slightly jiggly in center.
08 - Heat milk, cream, and vanilla to simmer. Whisk yolks and sugar until pale. Gradually whisk hot milk into yolks. Return to pan, cook over low heat until thick enough to coat spoon. Strain and keep warm.
09 - Dust soufflés with powdered sugar. Serve immediately with warm crème anglaise.