Split the vanilla pod and scrape out the seeds. Add both the seeds and pod into the milk, and bring to a boil
In a separate bowl, whisk together the sugar, egg yolks, and custard powder until smooth.
Temper the yolk mixture by gradually adding the boiling milk while whisking constantly.
Return the mixture to the pot and cook over medium heat, whisking until it thickens into a glossy pastry cream.
Remove from heat, discard the vanilla pod, and cool the pastry cream to 35–38°C.
Add the butter and blend to form a smooth, stable emulsion.
Cover with film (touching the surface) and chill until ready to use.