In a saucepan, heat cream to a boil. Remove from heat; whisk in chocolate until melted and smooth.
In a medium bowl, whisk egg yolks with sugar. Gradually whisk hot cream mixture into eggs. Whisk in vanilla. Divide mixture among 4-six oz (175 mL) ramekins. Place ramekins in a large shallow pan; pour boiling water into the pan to come halfway up sides of ramekins.
Bake for 25 to 35 minutes or until edges are set but centres still jiggle slightly. Remove from water; let cool on a rack. Cover and refrigerate for about 2 hours or until chilled or for up to 24 hours.
Divide graham crackers evenly among ramekins; top with marshmallows. Place on a baking sheet. Broil for 10 to 45 seconds or until marshmallows are lightly golden and starting to soften. Drizzle chocolate over marshmallows. Serve immediately.