Using the tip of a small knife, split the Vanilla Pod lengthways and open out. Cut in half to make short lengths.
Put the milk in a small saucepan and heat until just below boiling point. Remove from the heat and add the Vanilla Pod halves. Leave aside for 30 minutes to infuse, then scrape out the tiny black seeds from the centre of the pod halves into the milk; discard the pod halves.
In a heatproof bowl, whisk the egg yolks with the caster sugar until thick and pale. Gradually whisk in the Vanilla infused milk.
Place the bowl over a saucepan of barely simmering water and cook, stirring occasionally with a wooden spoon, until the mixture thickens sufficiently to coat the back of the spoon. Remove from the water and cover the bowl with a piece of cling film to prevent a skin forming. Leave the custard to cool completely.
Once cool, whip the cream until thick, then add the custard and continue to whisk until the mixture is thick enough to hold its shape – this may take a few minutes.
Spoon into 6 small freezer-proof serving glasses or dishes and put in the freezer for about 1 ¼ - 1 ½ hours until softly frozen.
To make the chocolate curls to decorate your mousse, melt the White Chocolate in a heatproof bowl over a pan of barely simmering water, and then spread over a small board. Leave in a cool place until firm but not completely solid (try to avoid refrigerating the chocolate unless the air temperature is very warm). Using a butter curler or sharp knife, scrape across the surface of the chocolate so that the chocolate rolls up on itself forming a curl of chocolate. Place on a board lined with baking parchment and leave in a cool place to firm up before using as a decoration.
Remove the mousse from the freezer. Serve immediately topped with chopped pistachios and White Chocolate curls.