Bring cream and half-and-half to a boil in a large heavy saucepan. Reduce heat to low. Add finely chopped chocolate and whisk until melted and very smooth. Remove from heat.
In a large bowl, whisk yolks until they are uniform in color. Gradually whisk in half of hot chocolate mixture. Whisk until very smooth. Once the eggs are slightly warmed in this way, gradually add the rest of the hot mixture, whisking all the while. Strain, if desired.
Divide custard among 8 (3/4-cup) custard cups. Place cups in large baking pan. Add enough hot water to pan to come halfway up sides of cups. Bake until custards are set, about 50 minutes. Remove from water; chill 3 hours.
To finish, top custard with a layer of mini marshmallows. Use a kitchen blowtorch to evenly caramelized the tops of the marshmallow and garnish with graham crackers.