To make the custard, whisk the egg yolks and sugar with electric beaters until the mixture is pale and thick enough to hold a trail when you lift out the whisk. Bring the cream and vanilla pod just to boiling point in a pan, strain. Add the hot cream to the sugar and egg mixture little by little, until it is all mixed in. Put the bowl over a pan of very hot but not boiling water, and cook, stirring until the mixture is beginning to thicken a little, and feels hot to the touch. Be careful not to overcook or the eggs will scramble. Now add the armagnac, but don’t keep cooking or you’ll cook off the alcohol. Cool over a bowl of ice.