To make the custard, whisk together the egg yolk, sugar and cornflour in a small bowl. Heat the crème fraîche and the split vanilla pod in a small saucepan until just below boiling point. Pour the hot crème fraîche over the egg mixture, stirring constantly. Return the custard mixture to the pan over a low heat. Stir constantly until the custard thickens and just starts to bubble. Remove from the heat, pour into a bowl, discard the vanilla pod and cover the surface of the custard with clingfilm to prevent a skin forming.