1. Preheat oven to 300ºF. Whisk egg yolks and vanilla together in a medium mixing bowl until combined.
2. Heat cream, chocolate and sugar, in a medium saucepan over medium heat, whisking frequently, until chocolate is melted.
3. Gradually whisk hot cream into egg mixture, until combined.
4. Pour mixture into ramekins and place them in a water bath (large pan filled with enough hot water to come up 3/4 way on to the ramekins.)
5. Bake for 50-55 minutes or until custard barely wobbles.
6. Remove from oven, remove ramekins from water and cool five minutes.
7. Preheat oven broiler. Sprinkle tops of custards evenly with graham cracker crumbs, followed by mini marshmallows. Broil custards a few seconds until marshmallows brown.
8. Cool for 30 minutes before placing in the refrigerator until chilled completely, about 1-2 hours.