Put the milk and vanilla pod into a heavy-based saucepan and heat until just starting to simmer. Remove the pan from the heat, stir in the instant coffee and grated chocolate and leave to infuse for 10 minutes.
Meanwhile, mix together the egg yolks, sugar and cornflour to a smooth paste in a mixing bowl or jug.
Remove the vanilla pod from the milk mixture. If you like vanilla pod seeds, scrape the seeds into the milk for a stronger vanilla flavour. Pour the warm milk onto the egg mixture, stirring.
Pour back into the pan and cook gently over a low heat, stirring constantly with a wooden spoon, until thick enough to coat the back of the spoon in a thin layer. Do not allow the custard to boil, and remove from the heat as soon as it has thickened. Serve the custard hot or cool.