1. Preheat oven to 325 degrees Fahrenheit.
2. Add chocolate to a glass or metal bowl. In a saucepan, warm heavy cream and milk until just bubbling, then remove from heat and pour over chocolate. Mix until mixture is smooth and chocolate is melted.
3. In another bowl, whisk together yolks, sugar, and vanilla until smooth. Slowly pour in chocolate mixture, whisking as you pour.
4. Pour mixture into four-ounce ramekins. Place in a water bath (a large baking pan with boiling water coming about halfway up the sides of the ramekin). Cover with foil and bake for 45 to 55 minutes.