1. Melt the crushed chocolate in a double boiler.
2. In a bowl, beat the egg yolks and sugar until the mixture turns white. Add the corn flour.
3. In a saucepan, bring the milk and cream to a boil. Pour over the egg yolk and sugar mixture while beating.
4. Return to the saucepan and cook while beating. Remove from heat as soon as it starts to thicken. Add the chocolate. Beat to obtain a homogeneous mixture.
5. Pour into ramekins lined with food film. Refrigerate for at least 4 hours.
6. Rinse the berries.
7. Sprinkle the top of the gratins with brown sugar. Caramelize for 3 minutes under the oven grill.
8. Remove from the oven and cover each gratin with berries. Serve.