Pastry Cream (Crème Pâtissière)

Heat 500 ml milk with a split vanilla pod. Whisk four yolks with 100 g sugar and 40 g cornstarch until pale. Pour the hot milk over the yolks while whisking, then pour it all back into the pan. Whisk over medium heat until it thickens and then let it boil for a full minute, whisking hard. That step cooks out the starch taste and stops the cream turning runny later. Off the heat, whisk in 30 g butter. Press plastic wrap onto the surface and chill it. Beat it smooth before you pipe it.