In a clean bowl, whisk together the egg yolks, sugar, lemon juice and vanilla pod over a saucepan of barely simmering water, until thickened.
Fold through the double cream into the bowl, and then gently pour the mixture into a tall glass.
For the spun sugar, in a small frying pan melt the sugar over a medium heat and when it turns into a deep golden colour, remove from the heat and use a fork to flick back an forth over a greased rolling pin.
To serve, gather the sugar strands together into a ball and top the posset with it.