Halve the vanilla pod and scrape out the seeds. Add both the pod and seeds to a pan on a medium-low heat, pour in the milk and bring just to the boil. Remove from the heat and leave to cool slightly, then pick out the vanilla pod. In a large mixing bowl, whisk the egg yolks with the sugar and cornflour until pale. Gradually add the warm milk, a ladle at a time, whisking well before each addition. Pour the mixture back into the pan and cook gently on a low heat for about 20 minutes or until thickened, whisking continuously. Delicious served with all kinds of crumble.