Heat the milk and vanilla until just boiling and then remove from the heat and if using a vanilla pod, remove the pod too.
Crack the eggs into a large bowl, add cornstarch and sugar and whisk until pale.
Add in a ladle of the hot milk slowly, whisking the whole time. Do this until all the milk is in the bowl and then transfer the whole mix back into the pan and put it on a medium to low heat.
Whisk it constantly until it thickens, should take 10-15 minutes.
Store in an airtight container and enjoy hot or cold (cold is best)
