Scrape out the vanilla pod and bring to the boil together with the cream.
Remove the pod.
Beat the egg yolks, sugar, vanilla sugar and cornflour until fluffy.
Pour the cream over the egg mixture while whisking. Pour back into the pan and bring to boil while whisking constantly. Remove from the heat when the cream has thickened. Pour the cream into a bowl and add the butter a little at a time while whisking. Refrigerate.
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