Melt the chocolate in the microwave for 2 mins, stirring halfway through, or over a pan of gently simmering water. Leave to cool a little.
Using an electric whisk, whip the double cream with the vanilla in a bowl until lightly whipped. Fold in the cooled, melted chocolate until fully combined.
Split the mixture between four small bowls or ramekins and serve topped with a dollop of crème fraîche. If you aren’t serving straight away, chill in the fridge and then add the crème fraîche just before bringing to the table.