1. Preheat oven to 300ºF. Put 8 6-oz. ramekins into a large roasting pan (they should fit without touching).
2. Put chocolate in a heatproof bowl. In a pan over medium heat, warm cream, milk and 1/4 cup sugar, whisking often, until sugar is dissolved and mixture is just simmering. Remove from heat, stir in vanilla and pour over chocolate. Let stand 3 minutes to melt chocolate; whisk until smooth.
3. In a second bowl, whisk remaining sugar and egg yolks until well combined and pale yellow. Whisking yolk mixture vigorously, slowly pour in about 1 cup chocolate mixture. Pour in remaining chocolate mixture, whisking well. Strain mixture through a fine-mesh sieve into a large measuring cup.
4. Pour mixture from measuring cup evenly into ramekins. Pour enough hot water into roasting pan to come halfway up sides of ramekins. Carefully transfer pan to oven and bake until custards are just set (they will jiggle slightly in center), 25 to 35 minutes. Remove ramekins from water bath and let cool to room temperature on a wire rack. Cover loosely with plastic wrap and chill until cold, at least 1 hour. Serve topped with dollops of whipped cream, if desired.