Preheat the oven to 230°C/210°F/Gas 8 and lightly grease 4 x 150ml pudding tins.
Break the dark chocolate into pieces and melt with the butter over a pan of gently simmering water, then set aside.
Meanwhile, whisk the eggs, egg yolks and sugar together until thick and creamy. Whisk in the warm melted chocolate and vanilla extract. Stir in the ground arrowroot to make a smooth batter.
Divide the mixture between the prepared tins and cook for 10 minutes until the sides are set but the middles are wobbly.
Immediately invert the puddings onto warmed serving plates, but leave in the tins for two minutes before removing. Serve with a dollop of crème fraîche.