01 - Preheat oven to 400°F. Generously butter four 7-ounce ramekins and dust with sugar, tapping out any excess. Set aside.
02 - Set up a double boiler with a heatproof bowl over barely simmering water. Add chopped chocolate and butter, stirring until completely smooth and melted. Remove from heat and let cool slightly.
03 - In a medium bowl, whisk together egg yolks, 1/4 cup sugar, vanilla extract, and salt until pale and creamy, about 2 minutes. Stir in the cooled chocolate mixture and flour until fully incorporated.
04 - Using a clean, dry bowl and electric beaters, beat egg whites until soft peaks form. Gradually add remaining sugar while continuing to beat until stiff, glossy peaks form.
05 - Gently fold one-third of the whipped egg whites into the chocolate base to lighten. Carefully fold in remaining whites, maintaining as much air as possible. Divide batter among prepared ramekins, smoothing tops. Run thumb around inside rim of each ramekin to ensure even rising.
06 - Bake for 16 to 18 minutes until soufflés have risen well and center shows a slight wobble. Do not open oven door during baking.
07 - While soufflés bake, combine milk, heavy cream, and split vanilla bean in a saucepan. Heat over medium until just below simmering. Meanwhile, whisk egg yolks with 1/4 cup sugar until pale and thick.
08 - Slowly pour hot milk mixture into yolks while whisking constantly. Return mixture to saucepan and cook over low heat, stirring continuously with a wooden spoon, until thickened enough to coat the back of the spoon. Do not boil. Strain into a clean bowl and remove vanilla bean if used.
09 - Remove soufflés from oven and serve at once. Pour warm crème anglaise over individual soufflés or serve alongside in a small pitcher.