Heat 500ml whole milk with the seeds from 1 vanilla pod in a pan. In a separate bowl, whisk together 1 medium egg and 1 medium egg yolk with 100g caster sugar; add 30g cornflour. When the milk has just reached boiling point, take off the heat and pour one-third into the egg mixture, whisking continuously. Pour the egg mixture back into the remaining hot milk. Return to the hob and bring to the boil. Whisk continuously as the mixture thickens, for just under a minute, then remove from the heat and stir in ½ tsp salt and 25g butter. Pour into a cold bowl and put a sheet of baking parchment on top to prevent a skin forming. The mixture will keep in the fridge for a few days. (The remaining mixture can be used in our Danish butter cake recipe).