Put the vanilla powder and milk in a saucepan and bring to a boil. Remove from the hob.

Whisk sugar, egg yolk and cornflour. Pour this over the milk while stirring. Bring back to the hob and warm on low heat while stirring, until cream thickens.

Remove from heat and add the butter while stirring. Pour the cream in a cold bowl. Put in the fridge to cool down. Stir occasionally  to avoid skin to form on the surface.